You need to sheet metal the bottom, a lot of the low end pop-ups just have exposed plywood and mice can chew right through that, do wheel wells also. Keep the trailer area clear of junk piles, trash and weeds. Get a cat. Put mice traps around the wheels and anything touching the ground and also use poison baits. At the end of the season clean your rig out completely leaving no food in it at all before putting it in storage mode.

Now the canopy if it has holes patch from the outside and use tent sealer around any edges. Use the same material and allow for shrinkage of the material patch.