Mice have chewed through the rubber seals under my slideouts. Since then I place mothballs on the exterior of the camper around the frame, slideouts, trim, tires, jack, etc. On the inside I use a couple different kind of dryer sheets. I know that neither of these is fully mouse-proof, but the combination has been working for me (knock-on-wood). The best of course is to seal up the camper fully, but when they chew through the seals, what can you do? I also FULLY empty out the camper, vacuum, mop etc. to try to get rid of any food/food smells.
I look at it my percentages/odds. While some of these things may be only 50% effective, the more things that are partially effective in combination, the better your odds. But reaching that 100% level is tricky!