I have an classic car that I store in the winter. When I first stored it, I never thought about mice getting in and did nothing - BIG MISTAKE there!! Not only did they use the interior as their bathroom, but I had acorn shells in every a/c vent, engine compartment, trunk -etc. It was a nightmare!!
I then learned to put 2 pie trays of mothballs under the vehicle - 1 front 1 back. sprinkled some around the engine area and a few in the trunk - worked like a charm! Yea, come spring it smelled a little like mothballs inside, but kept the windows
open for a week and it went away - sure beat the smell of mouse urine!
This winter we'll be storing our trailer in a rented barn on a farm and I'm already worried about mice getting in - we'll take everything out, cushions and all - inspect the underside of the trailer (to make sure there are not holes or cracks to enter) and put out a couple pie pans of mothballs just to make sure.
If anyone else has ideas I'd be sure glad to institute them!!