"RV manufactures drill over sized holes for EVERYTHING electric, water, etc. and they do not fill them up. Go out at night and turn the inside RV lights on and crawl underneath.....you will be shocked how many openings there are under your RV for every critter known to man to use to get into your RV!"
X2
some of the holes may be up over the tanks and are hard to reach, but they are there.
When all else fails, use a homemade self contained mouse trap. Thread a stiff straight wire through the ends of an aluminum beverage can so the can spins freely on the wire. Put a narrow band of peanut butter around the middle of the can and place the wire across the top of a 5 gallon bucket half filled with water or rv antifreeze. Place something such as a yard stick to make a ramp up to the rim of the bucket. Mice will walk up the ramp, cross the wire to the can and as they try to get to the peanut butter, the can will spin and dump them in the water. I am sure someone else deserves credit for inventing this, but it works great.