--- title: "Black and Gray Water Tank Repair and Replacement | RV" description: "Cracked polyethylene tanks, failed flush lines, blocked vent stacks and sagging straps on RVs and fifth wheels. $750 to $8,500+ at the Yorba Linda shop." focus_keyword: "black and gray tank replacement rancho santa margarita" canonical: https://ocrv.me/services/plumbing-repair/waste-water-tank-repair/ kind: subService updated: 2026-07-29 source: OCRV Center --- # Black and Gray Tank Repair, Welding and Replacement > Waste 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 ### Key facts | Item | Value | | --- | --- | | Price range | $750 to $8,500+ (5 to 35 labor hours at $260 per hour) | | Labor rate | $260 per hour (Mechanical and electrical department) | | Typical turnaround | 5 to 21 shop days (Tank lead time drives the schedule more than labor does) | | Hazmat handling | $45 flat (Applies where chemicals or LP are handled during the job) | | Written estimate | $150 (Credited in full against an authorized repair) | | Tanks fitted | Barker and Elkhart Plastics rotomoulded polyethylene | 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. **Sorting a waste tank complaint before anything comes off** | 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. > **price:** 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. > **insurance:** 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. ## Cost $750 to $8,500+, based on 5 to 35 labor hours at $260 per hour. Written repair estimate is $150, credited in full against an authorized repair. All work is performed in shop. ## 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. --- Source: [OCRV Center](https://ocrv.me/services/plumbing-repair/waste-water-tank-repair/). Last verified 2026-07-29.