Get in the trailer in the dark and shine a flashlight or a bright work light into all the corners of the cupboards, etc. and have someone outside underneath looking for the light coming through. Do the same from the outside in. One place we found on our 1705 was where the frame and the rear bumper met. There was a small opening between the frame and the interior just the right size for a mouse to enter, more specifically about a half dozen of them. They had a nest built in the rear bumper storage space and were running in and out from there. We placed traps and sticky traps in the cupboards and within a short time we had them all. There is a space that the plumbing runs in the length of the trailer and that was their main runway. Placing the traps near any openings seemed to be the best. DO NOT use poison unless you don't mind the smell of dead mice.
We got Chore Girl scouring pads and stuffed those into the openings, you could use steel wool also, mice will not chew on them, or so we think.
Supposedly dryer fabric softener sheets are supposed to repel mice and bugs. I don't know if it is proven but it doesn't hurt to stick some here and there.
Good Luck!