Black and Gray Tank Repair, Welding and Replacement
Written by RV Systems Lead, RV Systems and Electrical Lead, OCRV Center. Reviewed by Claims Coordinator
In shortWaste tanks fail at the mounting boss, at the flush line and at the vent, and access is what costs money. Work runs $750 to $8,500+ at $260 per hour depending on whether flooring lifts.
What does black and gray tank repair involve on an RV?
Black and gray tank work covers cracked polyethylene shells, flush line failures, blocked vent stacks, fouled level probes and the outlet flange at the bayonet. Owners from Rancho Santa Margarita bring units to Yorba Linda, where the underbelly opens on a lift and the tank is reached without cutting blind.
- A crack through a mounting boss is replacement, not weld repair
- Reaching the tank usually costs more than the tank itself
- Odour that peaks on hot afternoons is a vent path problem
- Rotomoulded tanks are unit specific and not returnable once ordered
- Price range
- $750 to $8,500+5 to 35 labor hours at $260 per hour
- Labor rate
- $260 per hourMechanical and electrical department
- Typical turnaround
- 5 to 21 shop daysTank lead time drives the schedule more than labor does
- Hazmat handling
- $45 flatApplies where chemicals or LP are handled during the job
- Written estimate
- $150Credited in full against an authorized repair
- Tanks fitted
- Barker and Elkhart Plastics rotomoulded polyethylene
Last verified
A holding tank spends its life carrying several hundred pounds of liquid that sloshes every time the unit changes direction, suspended from thin brackets under a floor that flexes. Nothing about that arrangement is generous. The tank shell itself is tough polyethylene and rarely the weak point. The weak points are the places where load concentrates: the mounting bosses, the strap lines, and the fittings glued or welded into a flat wall that was never meant to be a structural member.
The other half of this work has nothing to do with cracks. Fouled level probes, a flush line that has stopped working or will not stop working, a vent that was routed with a dip in it from new. Those are cheap findings on a page where the expensive findings live, and telling them apart before anything comes off the unit is the difference between a day and three weeks.
Waste tanks fail in three ways and only one is expensive
Start by narrowing which problem you have, because the range on this page is genuinely enormous and the reason is access rather than parts. Sensor faults are a nuisance: probes foul, a film bridges them, and the panel reports a full tank over an empty one. Vent faults are the odour complaints, and they get worse in warm weather because warm gas expands and finds whichever path is open. Both of those are hours of work with common parts.
The third failure is the shell, and it is the one that turns into three weeks. Polyethylene tanks crack where load concentrates, which in practice means out of a moulded mounting boss or along the line where a strap crosses. They weep when the tank has liquid in it and go quiet when it does not, so owners describe an intermittent drip that nobody could find. Reaching that tank means dropping an underbelly and often lifting flooring.
| What you observe | Most likely cause | Scale of the work |
|---|---|---|
| Full reading straight after a dump | Probes bridged with residue | Flush and clean, or fit an external sender |
| Odour worse on a hot afternoon | Vent stack with a low spot | Reroute for continuous rise |
| Water at the roof vent when flushing | Backflow preventer stuck open | Replace the flush fitting and nozzle |
| Intermittent drip under the floor | Crack from a mounting boss | Tank replacement plus strap correction |
| Weep around the bayonet with the cap on | Outlet flange gasket or bolts | Gasket and flange hardware |
Why getting to the tank costs more than the tank
A rotomoulded holding tank is not an expensive component. What it is, is buried. On most fifth wheels and trailers it hangs inside an enclosed underbelly between cross members, with the drop tube coming through the floor, the flush line and vent plumbed into its top face, and the outlet manifold hard against the frame. On many motorhomes it sits above a bay wall that has to come out. On some floor plans there is no path at all except through the floor, and that means lifting finish flooring and the decking under it.
That geometry is why the labor range runs from five hours to thirty five. The same tank, in two different units, is a day or a fortnight. It is also why we open the belly and look before quoting a shell, and why the estimate names the access path explicitly. An owner authorizing a tank replacement should know in advance whether their bathroom floor is coming up, because that answer changes the number by several thousand dollars.
On cost
Tanks are moulded to a floor plan pattern, not stocked. They are ordered on full payment and cannot be returned. That is the single largest reason this work carries a firm deposit at authorization.
Black tank flush lines, and the failure nobody expects
A black tank flush is a park water inlet plumbed to a spray nozzle inside the tank, with a backflow preventer between the two whose only job is to stop tank contents ever reaching a potable hose. When the preventer sticks open the tank fills continuously from the pedestal, and because the outlet cannot pass water as fast as a hose delivers it, the level climbs the vent stack and appears on the roof. Owners understandably read that as a blocked tank.
The other flush failure is the nozzle itself, which blocks with solids and stops rinsing. That one is quiet and it compounds: the flush no longer works, so solids build, so owners leave the flush connected for longer trying to clear it, which is exactly the condition under which a tired preventer gives up. We replace the fitting and clear or renew the nozzle together, because fixing one and leaving the other reproduces the same complaint in a month.
Vents, straps and the flange at the bayonet
Three parts of this system take almost no thought at build time and cause a lot of grief afterwards. The vent stack has to rise continuously from the tank top to the roof cap. Where a builder routed it around an obstruction with a dip, liquid sits in the dip and the vent becomes a trap, which means air cannot enter as the tank empties. That is what makes a dump slow to a trickle and a drain gurgle, and it is also the odour path back into the coach.
Support straps stretch and lose fasteners, and once one strap is slack the tank hangs on fewer bosses than it was designed to use. The outlet is the third: a bayonet flange bolted to the tank wall with a flat gasket, working every time a hose is twisted on. A weep there with the cap fitted is usually gasket and bolt tension, and it is one of the few items on this page that genuinely is a small job.
When a polyethylene tank can be welded and when it cannot
Polyethylene does weld, properly, with matched rod and controlled heat, and a good weld in a flat unstressed panel will outlast the rest of the tank. So a straight crack across a side wall away from any fitting or boss is a legitimate repair, and on a unit where access is brutal it is often the right economic call even though it is not what a parts catalogue would suggest.
What cannot be welded is a crack through a mounting boss, along a strap line, or at a fitting spigot. Those areas carry load or hold a threaded connection, and heat affected polyethylene in a load path fails again. We will say plainly which category a crack sits in, and where it is borderline we will weld it, fill it, load it and watch it for a day before we call it done rather than send an owner away hopeful.
Which tank failures a carrier will look at
A tank cracked in an impact is a covered loss and so is one holed by a road debris strike, which happens on the 241 more often than owners expect because a tank hangs low and the shoulder throws hardware. A tank that cracked from a strap that stretched over eleven seasons is maintenance, and no carrier pays for it. The finish and structural repair caused by the escape gets scoped with whichever category the failure falls into.
The distinction is made on evidence gathered before anything is dismantled. A fresh impact fracture has bright edges and often a matching scuff on the underbelly or a cross member. A fatigue crack out of a boss is dull, sometimes stained along its length, and has a stretched strap beside it. Those photographs go in the file at intake, dated, whether or not a claim is currently open.
On the claim
If the unit took a debris strike or a hard landing at any point, say so before the belly comes down. Establishing that link afterwards, with the tank already in a skip, is very difficult.
Which vehicles we do this on
Bunkhouse and standard fifth wheels lead this category by volume, because they carry the largest tanks and the longest tow miles with them full. Toy haulers add a garage floor above the tank bay that has to come up. Motorhomes hide tanks behind bay walls instead of underbellies. Mobile clinics and other commercial bodies carry gray tanks sized for sanitizing stations, and those units cannot operate at all while the tank is out, which changes how we schedule them.
What is included
- Underbelly opened properly, not slit
- The Darco fabric is released at its fasteners and folded back so it can be reinstalled tight. A knife cut across an underbelly is how a coach ends up with a flapping belly pan and a wet insulation blanket.
- Tank fill and load test
- Suspect tanks are filled with clean water and observed under load rather than judged empty on a lift. A boss crack only weeps when the weight is in the tank.
- Camera inspection through an existing opening
- A rod camera goes in through the toilet or the vent so a suspected blockage and a suspected crack are told apart before any bracket is undone.
- Vent path verification to the roof
- The stack is traced end to end and checked for a continuous rise. A vent is the one part of this system nobody thinks about and it causes half the odour complaints.
- Strap, cross member and support correction
- Whatever let the tank hang wrong gets fixed alongside the tank. Hanging a new shell from tired straps buys a season.
- Fill, dump and flow test before releasewhere applicable
- The system is filled, dumped and watched at the outlet, and the flush is run to full pressure with the vent observed from the roof.
How the work runs
Separate the cheap findings from the expensive ones
1 to 2 hours
Probes, vents, flush fittings and outlet flanges are all reachable and all inexpensive. A cracked shell is neither. We test in that order deliberately, because owners arriving convinced they need a tank frequently need a flush fitting and a probe clean.
Open the underbelly and look at the mount
2 to 4 hours
The unit goes up, the fabric comes back at its fasteners, and the tank, straps and cross members are inspected together. Boss cracks, stretched straps and rubbed drop tubes are all visible at this point and none of them are visible before it.
Fill and load the suspect tank
1 hour
Clean water goes in and the shell is watched under real weight. This is the test that distinguishes a boss crack from a fitting weep, and it is the reason a tank diagnosis cannot be done by looking at a dry tank on a hoist.
Order to the pattern, not to the catalogue
1 hour
Rotomoulded tanks are specific to a floor plan and are not returnable, so dimensions, fitting positions and boss locations are measured off the unit and checked against the moulder's drawing before anything is ordered.
Fit the tank and rebuild the support
6 to 26 hours
New shell, new straps where they are tired, corrected cross member support, fresh solvent welds on the drop tube and a new outlet gasket. Access panels or flooring that came up gets rebuilt as finish work, quoted separately.
Fill, dump and prove the vent
2 hours
Tanks are filled with clean water, dumped through the bayonet while somebody watches the outlet, the flush is run at full pressure, and the vent is observed from the roof to confirm air is moving the right direction.
What this costs
$750 to $8,500+
5 to 35 labor hours at $260 per hour, mechanical and electrical.
- Hazmat and disposal $45 flat when chemicals, refrigerant or LP is involved.
- Parts at $100 or less carry 100 percent markup. Parts over $100 carry 35 percent. Special order parts require a full non refundable deposit at order.
- Sales tax of 7.75 percent applies to parts and materials. Labor is not taxed.
- A 50 percent deposit is taken at authorization on any job over $2,000, with an additional 25 percent when parts arrive on any job over $10,000. Final balance is due at pickup.
- A flush fitting, probe clean and vent reroute sits at the bottom of this range.
- A black tank replaced on a coach where flooring has to lift sits at the top.
- Rotomoulded tanks are made to a unit pattern, are not returnable and are ordered on full payment.
How estimating is billed
- Written repair estimate: $150, credited in full against an authorized repair.
- In depth diagnostic, scan and programming: $285 per hour, one hour minimum, credited in full against an authorized repair.
- In shop pre purchase inspection: $400 to $1,200 depending on unit size and systems count.
- Insurance walk ins are accepted during posted hours. All inspection and estimating work is performed in shop at the Yorba Linda facility.
What you will notice first
- The gray tank reads full again within a minute of dumping at the station
- Sewage odour inside the bathroom that gets stronger through a hot afternoon
- A dark stain or a steady drip on the underbelly fabric beneath the tank
- The toilet drains slowly and the shower gurgles while it does
- Running the black tank flush sends water spraying out of the roof vent
- The bayonet outlet weeps around its flange even with the cap fitted
- One side of the tank visibly sits lower than the other from underneath
- A dump that starts fast and then slows to a trickle every single time
How this fails, and what we do about it
| Failure mode | How we diagnose it | The fix |
|---|---|---|
| Crack radiating from a mounting boss | Wipe the shell dry, fill the tank with clean water and watch. A boss crack weeps under load and closes when the tank is empty, which is why owners report an intermittent drip. Straps almost always show a stretched fastener nearby. | Replacement. The boss carries load and a weld through it will crack again on the first long tow. We correct the strap and cross member support at the same time, because the boss failed for a reason. |
| Flush line backflow preventer stuck open | Water appears at the roof vent when the flush is connected, or the tank refills on its own after dumping. Isolating the flush inlet and watching the level confirms it inside ten minutes. | Replace the flush fitting complete with its preventer and check the internal spray nozzle, which is frequently blocked in the same units and is why owners started leaving the flush running. |
| Trapped vent stack | Drains gurgle, the bathroom smells worse in warm weather, and a dump slows to a trickle. A camera up the stack or a smoke test finds the low spot holding liquid. | Reroute the vent with continuous rise to the roof and refit the cap. Adding chemicals to a trapped vent masks it for a week and changes nothing about the routing. |
| Bridged level probes | Full reading straight after a verified empty dump. Confirmed by dropping a camera through the toilet or the vent and seeing a clean tank with a coated probe. | Flush, clean and retest the probes. Where the probes are eroded we fit an external sender strip instead, which reads through the wall and cannot foul. |
Why it happened
- Road load on a full tank working a crack out from a moulded mounting boss, which is the thinnest and most stressed part of the shell
- A support strap that has stretched or lost a fastener, transferring the tank weight onto two bosses instead of four and cracking them in sequence
- A flush line backflow preventer stuck open, so park water feeds the tank continuously and rises up the vent stack instead
- A vent routed with a low spot at build time, which fills with liquid and turns the vent into a trap that pushes odour back into the coach
- Probe fouling, where paper fibre and residue bridge the sensor and the gauge reads a film as a full tank
- An ABS drop tube joint that has let go at the solvent weld, usually where the tube passes through the floor and has been flexed by a rocking toilet
Parts and brands we work with
- Barker and Elkhart Plastics rotomoulded polyethylene tanks built to unit specific patterns
- Valterra bayonet outlets, flange gaskets, Twist On caps and Bladex blade valve bodies
- Camco and Valterra flush line fittings with integral backflow preventers
- Horst Miracle Probes and SeeLevel II external sender strips for level sensing
- ABS drop tubes, closet flange adapters and Oatey solvent cement for the drain side
- Lippert Darco underbelly fabric, tank support straps and cross member hardware
What not to try yourself
- Cutting the underbelly fabric open to have a look. The belly pan is part of how the floor stays dry and warm, and a slit one collects road spray for the rest of the unit's life.
- Sealing a boss crack with a two part epoxy or a patch kit. The boss is a load path, the patch is not, and the crack reopens on the first freeway grade.
- Leaving the black tank flush connected and running to clear a blockage. If the preventer has failed, park water goes into the tank faster than the outlet can pass it and it comes out the vent.
- Dropping tank straps without supporting the shell first. A part full tank falling ninety millimetres onto a cross member cracks in a place that was previously sound.
Which vehicles we do this on
Vehicles we perform Waste Water Tanks on
BunkhouseBunkhouse fifth wheel repair deals with the second entry door cutout weakening the rear sidewall, bunk room floors over the rear overhang, and a half bath crammed into a rear cornerStandardStandard fifth wheel repair concentrates on the pin box and upper deck welds, the floor at the step up transition, the front cap above the pin and the roof front radius seamToy Hauler TrailerA toy hauler travel trailer carries its garage behind the axles on a straight frame, so rear loading removes tongue weight and makes the rig sway sensitiveClass CClass C motorhome repair centers on the cabover bunk, the joint where the coach body meets the factory van cab, and the stick built or laminated house behind itDiesel PusherDiesel pusher repair means working on a rear engine coach built on a welded Freightliner XC or Spartan rail, with a full body painted fiberglass shell and large basement baysMedical ClinicMobile medical clinic repair covers the exam room shell, the wheelchair lift opening in the floor, the welded clinical floor seams, the lead lined radiology wall and the roof condenser curbsI refuse to quote a tank off a photograph, and it annoys people. The reason is that half the units that arrive here for a cracked black tank do not have one. They have a flush fitting that stopped closing, or probes wearing a coat of paper, or a vent with a belly in it that somebody routed badly in 2011. Those are hours, not weeks. When it really is the shell, it is nearly always cracked out of a mounting boss and it is nearly always because a strap gave up first, so I will not hang a new tank without fixing what let the old one hang wrong.
Appendix: questions and answers
- Why does running my black tank flush push water out of the roof vent?
- The backflow preventer in the flush fitting has stuck open, so pedestal water enters faster than the outlet can pass it and the level climbs the vent stack. Disconnect the hose and dump. We replace the fitting complete and clear the internal spray nozzle, which is usually blocked in the same unit.
- Can a polyethylene holding tank be welded, and does the weld actually hold?
- In a flat unstressed panel away from fittings, yes, with matched rod and controlled heat. Through a mounting boss or along a strap line, no, because that area carries load and heat affected material fails again. We fill and load test a borderline weld for a day before calling it finished.
- What does a sagging tank strap do to everything plumbed above it?
- It shifts the whole tank weight onto fewer mounting bosses, which cracks them in sequence, and it lets the shell move enough to work the drop tube joint and the vent connection loose. That is why we correct straps and cross member support alongside any tank we fit rather than after.
- How do you tell a fouled level probe from a real blockage inside the tank?
- A camera on a rod, through the toilet or the vent. A bridged probe shows a clean tank floor and a coated sensor. A blockage shows solids standing above the outlet. Guessing between the two is how owners end up buying chemicals for a sensor problem for two years running.
- Why is the bathroom odour so much worse in the afternoon than in the morning?
- Warm gas expands and takes whichever exit is open. If the vent stack has a low spot holding liquid, that exit is the drain rather than the roof, so the smell arrives indoors as the day heats up. It is a routing fault, not a cleaning fault, and chemicals will not change it.
- Will my bathroom floor have to come up to change the tank?
- On some floor plans, yes, and we tell you before you authorize rather than on day nine. Where the tank sits between cross members inside an underbelly it usually drops from below. Where a builder trapped it above a bay wall or under fixed cabinetry, the path is through the floor.
Open a file on this repair
Tell us the vehicle, what happened and whether a claim is open. We will tell you what the scope looks like and what it takes to get you back on the road.
Or call (949) 799-3387