I have a fairly new trailer on some property that we visit often and it was always the awful anticipation of unlocking the door and finding the mice mess everywhere. I spent days under the trailer blocking off suspected entries, I would take a plastic soda bottle, cut it in half so the usable piece was the screw top, neck and partial bell. Slit that piece and the screw top so it will wrap around a pipe and act as a rat guard similar of what I saw on coconut trees out in the south pacific. Every penetrating water line, gas pipe and wire loom now has one but that just slowed them down.
Some time ago I read or heard that mice travel on theirs or others urine scented "highways" that generally lead to a food source or nesting/nesting supplies area. Since we usually only visit our dream property a couple of times a month, I had to come up with something to defile their scent trails under the trailer. I did wrestle with my concience for a while before I took this action so before I tell you what I did (going on two years now and we haven't had a single mouse invasion since) I must apologize to the green side of this forum-no, not the John Deere folks unless some of them are double green.
Bleach. Good old Clorox UNSCENTED bleach, full strength in a garden sprayer.
It doesn't take much at all, one quick pass around and under the trailer and I'm done. I do that just when we are packed up and leaving the property. I've since gotten an all plastic dedicated sprayer with no metal parts as bleach will over time clog up a metal wand so it must be thoroughly cleaned with fresh water after each use.
Our place is about ten miles west of Yosemite NP and we do respect those that have lived there before us but I draw the line at vermin. Here are a few of our neighbors at about seventy five yards out from our door: