If you look closely at the underbelly of your RV, then you will find all sorts of openings through which mice can enter. As you know, all they require is a very small "door" to find their way inside. The underbelly has openings for jacks, plumbing lines, electrical wires, tank pulls, etc. The area around your tires is especially susceptible. Many of these exterior holes can be plugged up using canned expanding foam.
There are also numerous holes inside your unit where wires and especially plumbing and water lines are routed from one compartment to another. These holes can be plugged using bronze wool and/or foam insulation or a combination of both.
Some members have placed various concoctions around their landing jack(s) and wheels to prevent mice from getting up and inside your unit.
Don't forget to remove all food sources from inside and place some bait or traps about your RV.
2008 Cougar 5th Wheel 27RKS
2005 2500 GMC Duramax