I sealed mine up using a couple cans of "Great Stuff" and a lot of work. I found openings for gas lines, electrical, and plumbing. Most of these were inside false walls in cabinets or cabinets that were screwed shut and not meant to be used. The drain line for the tub also went down through the floor under the tub itself - that one was the most difficult to get to and seal. Also, if the metal or plastic pieces inside your wheel wells don't mate perfectly with the walls/floor, seal those openings too. Basically, it's a rectangular box - open up anything that is blocking you from seeing the exterior wall and seal up all openings no matter how small. (Mice can get through a hole a quarter inch in diameter.)
When winterizing I also cover the vents for the refrigerator and the furnace with aluminum foil. (Yes, they could easily get through it if they want to, but it ends up mice really don't like the stuff and don't try.)
I had mice in my trailer the first winter I owned it, did this the following summer, and haven't had a single mouse in the trailer since.
Now if I could just figure out how to keep them out of my van!
