I used to have mice problems big time. One thing that I found was crucial was not to bait them. Do not set out traps with something like peanut butter. They can smell that 100 yds. away and come running to try to get into your rig to get it. They will chew their way in.
The only thing that I have found to work is moth balls. When in storage in an enclosed rental storage shed there was old hay leaves where someone had stored hay in the rental unit. I raked as much of it out before winter storage, then put moth balls on the feet of each jack or around each tire, anything that touched the ground. In the spring, there had been no mice in the rig and I could see mice trails through the dirt and hay under and all around it.
When camping for any more than 4-5 days, I also put the moth balls in the same places. I have had no trouble in 3 yrs. I also keep a box of moth balls in the front storage compartment in a zip lock bag all the time. Every so often, I get only a slight whiff of the moth balls.
I absolutely hate that smell, but it is not bad enough to give up the mouse protection. My wife has never said a word about it. I don't think she even knows that's what I do. She is not scared of mice in any way, except she hates the droppings.