Mice help

[wdk450]

This might not work if your trailer is "actively" being used; but for longer term storage, we have done the following with great success.
We bought a roll of 15" wide aluminum flashing.
Cut a length to go around your tires, standing up on edge.
A clamp of some sort or a couple of small bolts with wing-nuts to hold the overlapping edges together.
This quite effectively blocks the critters from climbing up the tires and gaining access to the frame; and thus any openings. If you block access to the frame, there is zero chance of them getting inside, as they must first find a path from the ground up; your tires are the only path up (unless you have your power cord plugged in, I guess). And assuming there is no access point going up your front landing legs.
This has worked quite successfully for us for several years.

Out here in suburban California I have seen the wide bands of metal flashing applied to the trunks of date palm trees to keep the mice, rats, and squirrels out.