I had a 1996 21' Sovereign
that had a mouse invasion. I tried the ultrasonic device. Went in the door one day and right in front of the ultrasonic emitter was a mouse eating something. Got rid of those devices and started looking for entry holes. Found openings in the front of the A frame. Wadded up chicken wire and stuffed it in there. Also found a hole where the electric power came in. Filled the hole with black gummy caulking. Those seemed to stop the entry of the mice.
My current 2007 27'FB had an invasion also. The only opening I could find was through the utility drawer under the rear bumper. I could not find a way to plug that one so just leave the old fashion mouse traps set in the drawer when the trailer is in storage. That seems to work also.