IMO, seal the trailer to keep them out in the first place. I went all over our trailer and sealed every spot where a pipe or wire went through the floor. Some of these can be hard to get at - requiring removing false panels, and crawling around underneath it, and my trailer had one underneath the tub that proved tricky. I did that four years ago and haven't had a single mouse in the trailer since. (They still nest in the wheels, spare tire, and bumper. That last one proved interesting one year when I went to use the sewer hose...)
I used expanding foam (Great Stuff) and it's worked great.