A toilet that won't flush is an inconvenience. One that won't stop running is quietly draining your fresh water tank into the black tank. Either way, it's almost always the same fix.
An RV toilet that won't flush is an inconvenience. An RV toilet that won't stop running is a crime scene waiting to happen — slowly draining your fresh water tank into the black tank while you sleep. Either way, the fix is almost always the same component: the flush seal.
RV toilets are mechanically simple. There are no tanks sitting behind them like a residential toilet, no float valves, no flappers. Water comes in through a valve when you push a pedal or button, passes through the bowl, and exits through a rubber seal at the base. When something goes wrong, it's almost always that seal, the water valve, or the cable connecting the pedal to the valve. Let's narrow it down.
Won't Flush At All
If pressing the flush pedal does nothing — no water, no movement:
Check your water supply first
Is the pump on? Is city water connected? Is the water heater bypass valve in the correct position? We've gotten calls about "broken toilets" that turned out to be low tank water or a pump switch that got bumped. Rule it out in 30 seconds.
Test the water valve
The water inlet valve is usually behind the toilet or accessible from underneath. If water is running everywhere else but not to the toilet, the valve may be stuck closed or the supply line kinked.
Check the flush pedal mechanism
On foot-pedal models (Thetford, Dometic), there's a cable or rod connecting the pedal to the water valve. These corrode and disconnect. Press the pedal and watch if anything moves on the valve side — if the pedal travels but the valve doesn't open, the linkage is broken.
Inspect the flush seal
The rubber seal at the base of the bowl controls the flap that opens and closes to the black tank. If it's torn, warped, or full of debris, the flap may not open even when the mechanism is working. You'll be able to see it when you look straight down into a dry bowl.
Won't Stop Running (Constantly Trickling)
This is the one that gets expensive if you ignore it. If water is continuously running into the bowl — even slowly — you're dumping fresh water straight into your black tank around the clock.
- A worn or debris-covered flush seal is the most common cause. If the rubber seal doesn't create a complete water-tight barrier, water seeps through. Sometimes cleaning the seal face with a damp cloth is enough if there's mineral buildup. If the rubber itself is cracked or deformed, it needs replacement.
- A stuck or faulty water inlet valve can also cause a slow run — the valve isn't fully closing after each flush.
- Check if the pedal is physically stuck in a partially depressed position — floor mats, dirt, or a bent bracket can prevent full release.
The seal replacement is a legitimate DIY job
Thetford and Dometic both sell seal and gasket kits for around $15–25. The process involves draining the bowl, removing two or four bolts at the base, lifting the toilet off, replacing the seal, and reinstalling. Total time: 30–45 minutes with basic tools. The part number is usually on a sticker under the toilet or in the manual. Get the kit specific to your model — universal seals often don't seat correctly.
The Black Tank Angle
Sometimes what looks like a toilet problem is actually a black tank problem. If you have:
- Odors coming up through the bowl even when the flush seal is closed — the seal may be fine but the black tank vent is blocked, creating back-pressure.
- Gurgling sounds from the toilet — same cause. Check the roof vent pipe for debris, bird nests, or a failed vent cap.
- Slow draining after flushing — a full or nearly full black tank, or a clog at the tank inlet. Don't keep flushing to "push it through" — that doesn't work and it makes the problem worse.
Common Question
How often should I replace the RV toilet seal?
There's no fixed schedule — some seals last 5+ years, others fail in two. The variables are how often you use it, your water chemistry (hard water accelerates deterioration), and whether the toilet ever sat dry for long periods (rubber seals dry out and crack). Check it annually: look for any cracking, hardening, or uneven surface around the seal ring. If it looks or feels brittle, replace it before it fails mid-trip.
When the Whole Toilet Needs Replacing
Most RV toilets are not worth major repair investment once the porcelain (or plastic bowl) is cracked, the base casting is broken, or the flush mechanism has failed multiple times. A replacement Thetford or Dometic runs $150–$350 depending on the model and whether you want a low-profile or standard height. Installation is a 45-minute job. If you're chasing the same repair for the third time, a new toilet is a better use of money than a third seal kit.
RV toilet issues are annoying but rarely complicated. Seal kit, water valve, linkage check — that covers 90% of what we see. If yours has been acting up for more than a few weeks or you're dealing with chronic odors that don't respond to tank treatment, give us a call and we'll sort it out on-site.
BossBros RV Team
Redding, CA