I had the mouse problem two years ago with my 2014 Rockwood. Imagine my chagrin when I got under the trailer and found numerous mouse entry holes. After sealing up the most obvious holes and still getting mice in the trailer, I decided to do something with the slide out driveshaft hole. The hole for mine was about the same as the "before" photo Carnation Sailor put in his posting.
I ended up cutting up a five quart oil jug and using the flattest part of the plastic jug to make a cover. I used a spade drill bit to cut a somewhat oversized hole in the plastic, cut the plastic into a rectangle that would fit up against the trailer frame and made a final cut from the round hole to one edge of the plastic. That last cut allows you to slip the plastic piece onto the drive shaft.
Once I determined my plastic cover was the size I wanted, I thoroughly cleaned the frame around the hole, applied silicone adhesive to the frame around the hole and glued my plastic cover in place.
So far so good - it's still in place. The hole in the plastic cover is large enough to allow for the movement of the slide out drive shaft but tight enough to keep out the critters.