When we bought our Class C, about 12 years ago we had several mice inside that we finally trapped. After this I lay down under the RV and searched for any spots that mice could get in. I found several. I sealed these holes; expanding foam, metal screening, sheet metal, etc. I also noticed that the extension cord I was using didn't really block the hole the cord ran through. I made a sheet metal cover for the hole that blocked access around the extension cord.
Haven't had a mouse in the RV since, at least 10 years.