To start all openings of any size from tiny tiny to larger need to be blocked somehow, foam etc., Openings caused by slide out mechanisms need to be blocked as best as you can.
Trailer has an opening for the power cable, switch it over to a external receptacle that blocks the opening.
This is the type of kit that would close that hole -
http://www.campingworld.com/shopping/item/50-amp-rv-detachable-power-conversion-kit/72103
If you have a fifth wheel you have to look at the two front leg compartments for openings, most of them open into the front cap area and that is bad, a quick way in. Foam all drain wire, and water pipe entrances into the trailer from the under carriage. Don't be afraid to take cabinet bottoms out, remove drawers and look inside cabinets or dressers you will be surprised at the openings you find.
Next August I will be surrounding my trailer with a ring of moth balls, dryer cloths inside along with sticky traps and peanut butter. I will have pugged every hole I can find and we will also be putting bay leafs in the drawers and any other thing I can think out.
This year we killed two mice in the trailer and found mouse poop on recliners, couch, inside couch drawer, on top of bedspread, in all the dresser drawers and all my cotton t-shirts were chewed up. My wife cleaned out mouse poop twice with in two weeks. We still have mouse poop in the 4 recessed living room fixtures. The four fixtures in the living room and in the kitchen don't work. I'm hoping the buggers didn't chew the wires.