Something is wrong with your black tank. Maybe it will not drain. Maybe the panel has read two thirds full since last summer no matter how many times you dump. Maybe there is a wet spot on the underbelly and a smell that follows you around the campground. These get worse while you wait, because a slow drip out of a cracked tank soaks insulation, rots subfloor, and turns a plastic repair into a floor job.
Mile High RV Works is the RV service side of Mile High Food Trucks. We build custom food trucks from bare chassis, so welding, fabrication, plumbing, and electrical all happen in our own shop with our own people. That matters here, because half of a black tank job is not the tank. It is cutting and re-supporting a coroplast underbelly, fabricating new tank straps, rerouting a drain line that never had enough fall. We serve the whole Front Range from one shop, so Denver, Boulder, Fort Collins, and Colorado Springs customers come to the same bay and the same techs. The rest of what we do is on the RV repair page.
Find out what actually failed first
“Black tank problem” covers about seven failures with wildly different price tags. Sorting them out takes ten minutes with a flashlight and a clear elbow on the outlet. Will not drain but the handle pulls normally: clog or vent. Handle will not move, or moves with no resistance: gate valve. Sensors lie but it drains fine: residue, not electronics. Sewage smell inside: toilet seal or roof vent. Drips on the underbelly: a crack, a bad flange, or a failed fitting. Quoting all of that as one lump is how people pay for work they did not need.
What black tank work actually costs
These are Front Range shop numbers for 2026, parts and labor. Independent shops in Colorado generally bill $130 to $180 an hour, dealers on the high end, mobile techs often above it. Full pricing is on the 2026 RV repair cost page, and you can ballpark your job with the repair cost calculator.
1. Sensor cleaning and tank flush ($150-$400)
Sensors almost never fail as hardware. Paper and solids cling to the probes, stay damp, keep the circuit closed, and the panel reads full forever. The fix is cleaning the tank walls, with an enzyme treatment or a rotating wand that pressure washes the inside. Price depends on whether we can reach the tank through the toilet or have to pull it.
2. Gate valve replacement ($200-$500)
The valve is a $25 to $50 part. Everything above that is access. On a travel trailer with the valve in open air behind the skirt, it is 45 minutes and four bolts. On a Class A with the valve buried in a sealed basement bay, it can eat four hours. A valve that leaks after dumping is usually the seal, not the blade.
3. Vent line and roof vent repair ($150-$450)
The vent stack is a 1.5 inch pipe from the top of the tank through the roof. Block it with cottonwood fluff or a mouse nest and the tank pulls a vacuum. The toilet drains slowly, the tank glugs, odor pushes back inside. Clearing it is cheap. A vent pipe that separated from the tank collar is not, because the underbelly has to come open.
4. Toilet flange and seal replacement ($200-$550)
The seal between the toilet and the tank flange goes flat and brittle with age, so you get water at the base of the toilet and odor no chemical will fix. The seal is a $15 part. It gets expensive when the ABS flange itself is cracked, because the flange is bonded to the tank top.
5. Clearing a pyramid clog ($250-$700)
Solids piled up under the toilet because there was never enough water to carry them away. It hardens into something closer to concrete than sludge. Soaking with water and enzymes clears the mild ones. Bad ones need a wand or flexible auger. Do not put a steel drain snake down a black tank. It will punch through a thin ABS wall.
6. Cracked tank patch or plastic weld ($300-$950)
A crack from a road strike, a freeze, or a broken strap can be repaired in place if it is reachable and the tank is otherwise sound. Cost comes down to whether the tank has to come out to get at it.
7. Full black tank replacement ($1,200-$3,800)
The tank is $200 to $600 off the shelf, more for an odd OEM shape that has to be fabricated. The rest is labor and it swings hard. A trailer with the tank strapped below the frame might be six hours. A motorhome with the tank boxed into a heated basement behind the generator and a slide rail can run past twenty.
| Situation | Repair | Replace |
|---|---|---|
| Single clean crack, reachable, no tank drop | $300-$650 | Not worth it |
| Crack requiring the tank to come down | $600-$950 | $1,200-$2,200, do this instead |
| Multiple cracks or brittle sun-aged tank | Not recommended | $1,500-$3,800 |
| Damage at a molded fitting or flange | Rarely holds | $1,500-$3,800 |
ABS or polyethylene changes the whole repair
Most RV holding tanks are ABS. Some are high density polyethylene. Know which you have before anybody touches it. If the drain pipe is glued into the tank fitting, the tank is ABS. If the pipe is clamped or threaded in, it is polyethylene.
ABS is the good news. It chemically welds, so the repair material dissolves into the parent plastic and becomes one piece. On a clean, prepped surface an ABS patch is permanent. Polyethylene is the bad news. Nothing glues to it reliably, no epoxy, no ABS cement, no marine adhesive. It has to be hot air welded with matching HDPE rod. We weld plastic daily on the fabrication side, so we do these in house.
Either material, one hard rule: repairs at a molded fitting, the flange boss, or a corner radius do not last. Those areas flex and carry load.
When replacement beats repair
Replace when the tank has more than one crack, when the plastic has gone chalky from UV, when the damage is at a fitting, or when the tank has to come out anyway. That last one catches people. If eight hours of the job is dropping and reinstalling the tank, spend $400 on a new tank rather than $150 on patch material.
What Colorado does to black tanks
Freeze splits. Most RVs are not four season. The underbelly is thin coroplast with a little batting, so the tank sits at whatever the outside air temperature is. A tank left partly full through a Front Range cold snap freezes, expands, and splits at a corner or a molded seam. Partly full is worse than full, because the empty space lets cold circulate. Waste sitting against a valve blade cracks the valve body the same way. Draining and flushing both tanks in October is the best thing you can do all year.
Dry air and early dumping. Colorado humidity is low enough that anything left on a tank wall dries hard instead of staying wet. Add the habit of dumping at a third full because the hookup is right there, and you get pyramid clogs and dead sensors faster than people do at sea level.
Outdoor winter storage. Rigs parked on gravel lots along I-25 take sun, wind, and freeze cycles all winter. UV makes plastic brittle, road grime eats valve hardware, mice find the vent stack. Most of the tank work we do in April traces back to how the rig sat.
Doing it yourself
Plenty of this is owner territory. Clearing a blocked roof vent, running a wand through the toilet, treating with enzymes to recover sensors, replacing a flange seal, and swapping an exposed gate valve on a travel trailer are weekend jobs with hand tools. Work with the tank empty, wear gloves, buy new seals with the valve.
Where it goes sideways is predictable. People snake a black tank with a steel auger and put a hole in it, turning a $300 clog into a $2,000 tank job. People patch polyethylene with epoxy, get two weeks out of it, and find the leak again after it has soaked the floor. People drop a tank on a jack with no support and crack it on the way down.
The honest line: if the fix is outside the tank and you can see it, try it. If it means cutting the underbelly, dropping the tank, or welding plastic, the cost of getting it wrong beats the labor you save.
Keeping it from happening again
Keep water in the tank. Never dump below two thirds full. Fill the bowl before you flush solids, and add a few gallons after every dump. Use an enzyme treatment instead of a formaldehyde chemical, because enzymes break solids down instead of masking odor.
Back flush every few trips, exercise both gate valves so the seals do not harden in one position, check the roof vent twice a year, and drain everything before storage.
Get a real quote
Call 720-209-2653 and bring the rig in. We do not quote black tank work over the phone, because the price is set by access and nobody knows the access until the underbelly is open. We do a diagnostic first: find the actual leak, check the valve and seals, scope the vent, confirm the tank material, and see what has to come off. Then you get a real number with parts and hours broken out.
We handle the whole system, not just the tank. A recent example is the Roadtrek 190 plumbing rebuild, where we replaced failed barb fittings and connectors throughout the coach for $1,320 including tax.
Frequently asked questions
Can a cracked black tank be repaired, or does it have to be replaced?
Repair works on a single crack in a flat reachable area, away from fittings, with the plastic still in good shape. Replace for multiple cracks, brittle sun-aged plastic, damage at a fitting, or any repair that needs the tank dropped anyway.
Why do my sensors still read full after I dump?
The probes are coated, not broken. Waste and paper stick to them and stay damp, keeping the circuit closed. Cleaning the tank walls fixes it in most cases. Only replace sensors after cleaning has failed.
How long does black tank replacement take?
A trailer with a strap-mounted tank below the frame is roughly a one day job. A motorhome with the tank boxed into a heated basement can take three to five days counting disassembly, the new tank arriving, and reassembly.
What is a pyramid clog and how do I know I have one?
A hardened cone of solids under the toilet, from dumping with too little water in the tank. Signs are a tank reading full when it is not, a toilet that backs up, and very little coming out at the dump valve.
Will freezing really crack a black tank in Colorado?
Yes, and we see it every spring. A tank left with liquid in it through a hard freeze splits at a corner or seam, and the gate valve is just as vulnerable. Drain and flush both tanks before you park it for the season.
Is a mobile tech or a shop better for this?
Mobile is fine for valves, seals, vents, and sensor cleaning. Dropping the tank or welding plastic wants a lift and fabrication tools.
Can I keep using the RV with a leaking black tank?
No. Waste soaking into insulation and subfloor costs multiples of the tank repair, and it is a health issue. Keep the tank empty until it is fixed.
Recent Comments