A few weeks back we also heard the feet of a mouse in the roof section that we cant get to. I did what I could to make the bottom of the unit to where they couldnt get in but I figured they got in by the LP tanks up front or on the other side through the storage area.
So off to the hardware store the next day and bought a dozen Victor traps. Baited them with peanut butter and placed them all over the rig and on the out side access points. Next morning had of culprit in the forward garage. It was a logical way to the roof section for my unit.
You have to figure mice need food and water and even if there in the roof they need to go out every once and a while. They just cant resist peanut butter! Better to head it off early. With colder weather coming in they want a nice warm out of the weather condo or winter.