An RV toilet that won't hold water in the bowl has a failed flush ball seal — the rubber seal between the bowl and the flush mechanism that holds water above the blade. Over time the seal dries out, cracks, or warps. Replacement is a 20-minute DIY job and a $15-$25 part. It's the single most common RV plumbing repair.
Most likely causes (in order of likelihood)
- Dried, cracked, or warped flush ball seal — by far the most common cause.
- Debris stuck on the seal — sometimes simply cleaning the seal restores function.
- Damaged or stuck flush ball/blade mechanism.
- Failed water valve allowing water to bypass the bowl entirely.
- Cracked bowl (extremely rare but possible after freeze damage).
Diagnostic steps (in order, free/cheap before expensive)
- Step on the flush pedal and look down into the bowl. The flush ball (the round seal at the bottom) should be smooth, even, and form a tight ring against the bowl.
- Inspect the seal for cracks, distortion, brittleness, or debris like toilet paper stuck to it.
- Try cleaning the seal first. Lubricate it with a few drops of plumber's grease or olive oil and run several cycles to see if it reseats.
- If cleaning doesn't fix it, identify your toilet brand (Dometic, Thetford, etc.) and order the matching flush ball seal kit. Brand-specific is important — they don't interchange.
- Shut off the water supply to the toilet. Flush to relieve any pressure.
- Remove the old seal by following the kit instructions. Most kits include a small retainer ring that needs to be popped off.
- Install the new seal with a thin coat of plumber's grease, reassemble, and test.
- If the toilet still doesn't hold water after seal replacement, the flush ball mechanism itself is worn — a full mechanism replacement is $50-$100.
DIY vs. call a tech
100% DIY. The seal kit comes with everything needed. The hardest part is identifying your exact toilet model — look on the side or back of the bowl base for a sticker.