An RV has more ways in than a house does. The underbelly is full of openings cut for plumbing and wiring, and any one of them can be a door.
Two guides here. One covers getting mice out of the RV and keeping them out. The other covers stopping bugs from getting in at all.
Traps on the travel routes, steel wool in the gaps, and care about what's safe around pets. What we learned clearing a mouse infestation out of our fifth wheel after six weeks in storage.
A trail of ants marched into our fifth wheel and Fabiola declared war. Here's the full defense we ended up with: vent screens, a perimeter spray, and a borax bait mixed at the right strength.