Those little buggers!
We had mice last summer, and it wasn't until I really went looking that I found a ridiculous number of places for them to get in. If there's a hole the size of a dime, they can get through.
Look for any penetration in to the trailer. In my case they came in around one of the drain lines to the grey tank where it penetrated the floor of the TT. The flooring material (the plywood subfloor and the linoleum) had been chewed away there which allowed very easy access. Two other points of access I found were the hot water tank (It was missing a rubber grommet) and the electrical cord. Also where the propane lines penetrated the floor for the furnace, water heater, and stove.
My trick to fill holes is stuff them with steel wool and spray foam insulation around it. If they try to chew through and get to that steel wool they usually stop and look for an easier target.
In winter storage we use a three-pronged attack to dissuade the little devils from coming in. Mint, Irish Spring soap (shaved in to small pieces), and bounce sheets. Since I plugged all the holes I could find and started with the mint/soap/bounce we haven't had a mouse in it.
This year we're in a new TT, so I'll have to look around to make sure there's no access for the little devils...God I hate mice. So much. Uggh...