Our first week hosting (second or third guests) a young couple 'on vacation' stayed in the small camper the entire weekend (went out once for an hour.) When they left we found the RV toilet piled with feces layered with toilet paper. They didn't know how to flush a RV toilet and rather than read the directions posted in the toilet seat they just . . . didn't.
Since then I demo the flush apparatus while talking the guests through the differences between this and a 'flush toilet', look the guests in the eye and say "we don't want to see any poop in here after you leave so please flush as long as needed using this foot pedal and this spray hose attachment from the nearby shower."
I also tell them if there is a problem to let us know right away so we can fix it before it compounds.
Nothing breaks the ice like saying the word poop several times to your new acquaintance. Especially if they don't speak much English and don't understand the word poop. Watch me pantomime pooping and flushing . . .
We discovered that we can say 'poop' or we can see poop. We would rather say it.
I also show the guests where we keep the cleaning supplies and suggest that if there is 'something you need to clean' here is what you can use.