The overnight test
Before bed, flush and then add water to the bowl to the normal line. In the morning, look: if the bowl is empty or noticeably lower, your bowl seal (the rubber blade seal at the bottom of the bowl) is leaking — and if water can leak down, tank odor is drifting up the same path all day.
If the water level held but the bathroom still smells, the seal is fine — move on to the vent stack and tank chemistry sections below.
Fixing a bowl seal that won't hold water
Start cheap: the seal is often just dirty. Close the blade, line the seal area with a paper towel soaked in hot water and dish soap (or a seal conditioner), let it sit an hour, then scrub the groove gently with a plastic brush — mineral scale and paper residue keep the blade from seating. Condition the rubber with a plumber-safe silicone grease afterward.
If cleaning doesn't restore the water hold, replace the seal. Most RV toilets sell the blade seal as a kit; the toilet unbolts from the floor with two closet bolts, flips over, and the seal swaps in minutes. While it's up, inspect the floor flange seal underneath — a distorted flange seal leaks odor at floor level and hides behind the toilet's base.
The vent stack: your tank's chimney
Every holding tank breathes through a vent pipe that runs up through the roof. When it's blocked — mud daubers, leaves, a cracked cap — tank gas can't escape upward, so it takes the path of least resistance: into the bathroom when you flush, or through a dry P-trap. A toilet that burps or gurgles when flushed is a classic blocked-vent sign.
Get on the roof and check the vent cap; run a plumber's snake or garden hose down the stack to confirm it's clear to the tank. While you're up there, consider one of the wind-driven vent caps that actively pull air up the stack — they turn a passing breeze into suction and are the single best upgrade for a chronically musty rig.
Tank chemistry and the water-first rule
A black tank needs water to work — waste breaking down dry on the tank floor is what makes the truly foul smells. Always start a fresh tank with a couple of gallons of water in it, flush with plenty of water, and use a quality enzyme or bacteria-based treatment so the tank digests instead of rots. In hot weather, double the water and the treatment; heat accelerates everything.
Skip the old formaldehyde chemicals — they kill the good bacteria along with the smell, stop the breakdown process, and many dump stations ban them.
Stuck on this one? Put Major in your pocket.
Tell RV Journey Genie your make, model, and symptom and get a step-by-step fix — by text, voice, or photo — before you ever call the shop.
Frequently asked questions
Why does my RV bathroom smell like sewer?
Three sources cover nearly every case: the toilet's bowl seal is letting tank gas past (test it — bowl water that drains overnight means a leaking seal), the roof vent stack is blocked so the tank breathes into the rig, or the black tank itself has too little water and dead chemistry. Test the seal first; it's the most common.
How do I fix an RV toilet bowl that won't hold water?
Clean the blade seal first — scale and residue often keep it from sealing. Soak with hot soapy water or seal conditioner, scrub the groove, and grease with plumber-safe silicone. If it still won't hold, replace the seal kit: unbolt the toilet, flip it, swap the seal, and re-set it with a new floor flange seal.
What should I put in my RV black tank to stop smells?
Water and biology. Start every empty tank with a few gallons of water, flush generously, and use an enzyme/bacteria treatment so waste digests. Avoid formaldehyde-based chemicals, which stop the breakdown process and are banned at many dump stations. In hot weather, use more water and more treatment.