We've had more than our share of the little buggers. Sealing our fifthwheel was hard but I finally closed off the last access and we haven't had any mice in it for almost a year now (not so with the mobilehome we live in, but we have a pretty good mouser and I place traps, together we get one or two per month).
One MAJOR access point on the fifthwheel was underneath: The long rod that moves the gears that moves the slide out had a 5" diameter hole through the frame for the 1" rod. Mice went through the hole, into the "basement storage" area, and then had free access via plumbing to the rest of the trailer. Some plumbers cloth screening fixed that.
Another place was the ends of the "beams" that the slideout rides on: both sides had matching holes in the frame and they would get into the basement storage through those holes. Some more plumbers cloth and some expanding foam closed that door.
The final entryways I found because the little guys aren't very clean and after awhile I noticed a "smokey" spot on the inside panelling where mice had been rubbing on their way in or out: it was at floor level, where the slide and floor and side meet. There was enough of a gap to allow them in. Since this part can't be closed solid, I use steel wool in a baggie, and stuff it into the dime-sized hole on each end of the slider right at the floor. If I forget to take it out when moving the slide (that happens often) it doesn't hurt anything, and I just retrieve the baggies and reinstall them.
These things are hard to describe but easy to show. If it'd help, I can post some pictures.
Phil
