See if you can seal up the holes/gaps in the trailer. Spread mouse poison all around the trailer, focusing on places they can crawl up, tires, jacks, stabilizers etc..... Sometimes Bounce original fabric softener sheets work by stuffing them in around the wire harnesses and such, but ya gotta pull 'em out before each ride. I would say you need to focus on the trailer though, keep 'em out to start with. Can you park the trailer on concrete or gravel? Sometimes it helps just to keep the trailer outta the grass.