Mice are territorial, so not likely to have more than 1 pair setup housekeeping in a camper. Gestation period is 19-21 days. That's when the trouble really starts.
It's easy to loose track of time. But if you check a stored camper for odor once a week, you'll probably be able to nip the problem in the bud with traps or sticky pads.
Never use poison unless you want to make your problem worse. Been there - done that.
My slideout is the weak point in my camper. I bring it in within about 6" of being fully closed and then spray the rubber seals and gaps from underneath with peppermint oil. I don't think it's so much that the mice don't like odor of the mint oil as it masks food odors in the camper.