Peppermint oil, dryer sheets, moth balls...nothing works.
You don't want mice? Seal up your trailer empty ALL food and if there is a sign of mice, set a trap.
Don't set a trap baited with food unless you can be there to check the trailer regularly. The food on the trap will attract the mice.
As for the other stuff, have a search of my posts on this. People have posted pictures of mice in a trap right beside cab fresh. Mice have frequently used dryer sheets for nesting material.
I did an experiment with mice and peppermint oil (yes the good kind) and it didn't bother them at all. They walked over it to get to food.
Sorry, but all the stories of the success with these methods is just plain luck.
You can't prove a negative.
Kinda like putting garden stones in my trailer, and then having no mice. Can I declare garden stones good mice deterrents?:)