We had the same problem last year. Had mice and couldn't see anywhere underneath where they could be getting in. THEN WE FOUND OUT WHERE.
Our larger slide out requires holes thru the side of the frame and one of the holes was so oversized, a squirrel could have crawled thru. Plugged it with some steel wool, set a few traps, and a couple of days later, no more mice.
2015 GMC Sierra 2500HD SLT 6.0L SRW
2017 Jayco Eagle 330RSTS (on the way)