Just had a weekend away and I'm noticing bad drain smells in the washroom (2006 Autocruise Starfire). Do the drains have traps in them to stop smells feeding back from the waste tank? If not is there any way to retro fit any?
If you have traps you should be able to see them under the sink etc. I don't know about a retro fit no doubt somebody else will help you on that. However I always empty my waste tank and put a bottle of cheap "Lidl" coca cola down all the plug holes and let it slosh about in the tank for a while. It cleans the tank up and gets rid of the smell
I now put a splash of bio washing liquid down the drain after every emptying. Not very much, about half a capful at most. But first I had to clean the waste tank out by filling it up completely with a large dose of bio added then letting it stand overnight. I had to do this twice to get my tank full sensor, an led, to finally go out. The inside of the tank was plastered with crud I think but eventually the waste water ran clear after a good flushing out and all now smells of roses.