Mice - Any advice on where to look for points of entry?

Ant place hoses, wiring, gas lines, etc enter the trailer from the under side. The enclose under belly is not likely to keep mice out. We had them come in ours where the intake for the water pump comes up from the tank. Wad up some window screen around the pipe and cover/fill with expanding foam. Mice can chew through the foam, but the screen stops them.