Some folks say it's hogwash, but I put Bounce dryer sheets in every nook and cranny in the camper. I also put them under mattresses, seat cushions, drawers, and other places. Don't use any fancy aromas (like Febreeze, etc) - just the plain, original Bounce. The word is that mice and other critters do not like the smell. I've parked a camper in a grassy area for nine years and never had a single problem with mice or other bugs (except those darn Asian beetles!).
I would NOT, however, use D-Con or other poison. If they crawl into an inaccessible space and die, the stink would be horrible and would last a long time, until you remove it - if you can find it. Glue boards or traps would be my preference.
I got this tip from an RV dealer and it has worked for me. (It was posted on their website.) Watch here: https://www.youtube.com/watch?t=15&v=B1y-o4DjM3I
2015 Jay Flight 32RLDS Elite
2012 Chevy 2500HD Crew Cab LTZ (6.6L Duramax/Allison)
Equalizer Hitch (1200# bars)