If some P traps aren’t used much, the liquid dries up and/or sloshes out on the road. We arent frequent showerers on trips, and I learned to pour a cup of water down that drain every so often to stop the gray tank back odor. Also you may have, like ours, special vent valve risers near and above the traps that keep the trap liquid from being sucked out by the vacuum created by the act of emptying your gray tank. If one of those little valves is befouled or stuck closed, emptying the tank after a park stay may mitigate the intent of its P trap and allow tank vapors up into the living space.
I too have read of the vacuum effect some like Bill have experienced on the road, but more where an open roof vent actually draws in vapors from a nearby roof tank fitting. Try to maintain inside ventilation such that there is no negative pressure inside on the road. I understand there are tank roof vents that are more effective than others at expelling fumes; but you may have a damaged one, or a partially plugged one so the tank(s) can’t vent to the roof properly, or road air is actually being pushed down the vent rather than up and away.
We’ve never had odor problems with our Splendide, even though it’s infrequently used, and I’ve never poured water or run a rinse cycle to fill its hard-to-access trap. I’m not certain why it’s never been an issue, given that I know others here have experienced a problem with their rarely used machines.
Joel
