I used a copper mesh to stuff around all pipes coming in from the basement and any place that it looked like a rodent might enter from outside.  It's pretty difficult to completely seal everything, especially around the battery compartment in our trailer.  If you have pets, be sure not to keep pet food in the basement unless it's in a rodent proof container.  Same with birdseed.  

We found evidence of mice a couple of times but the first one apparently moved back out since we didn't ever catch him and we found no more droppings.  The second one we heard in the middle of the night so we immediately got up and set a trap baited with peanut butter.  He took the bait almost immediately and was killed.  I thought he seemed pretty big and might be something other than a mouse but Bill says no.  I still say he seemed big and more brazen than I imagined a mouse would be.  He was bouncing around like a pinball while I was standing on the steps up to the bedroom.  Glad we caught him quickly cause I wasn't going to sleep knowing he was there.

Good luck!  They seem to like peanut butter.

Bob,

As others have said, the highest priority should be filling any external gaps (and getting rid of any mice that are in the RV). For sealing the basement, we used spray sealing foam, cooper mesh  or a combination of both on any basement gaps. Also look for any gaps that can be closed between the basement and the living area (e.g., we found a gap/opening to the basement in the corner of the entry area shoe nook (i.e., the shoe storage area that is below the entry close in many New Horizons)).

We also found placing LED rope lights under the RV to be helpful, when we are staying in more remote/natural areas. Some opinions vary on the effectiveness of rope lights, but after not using them at one remote location and seeing the difference after putting them out, we were convinced they made a difference.

Oh, I do have ultrasonic devices, fresh cab, mint oil soaked cotton balls in various places within the RV … but I think the real trick is sealing the basement as best as possible and it certainly does not hurt to seal areas between the basement and the living area. We will continue to use the LED lights under the RV in remote/natural locations, since you can never fully seal the basement (e.g., slide openings into the basement). We also place LED lights under the truck, in areas where we know there are mice or other similar animals (as was mentioned above, mice will chew on electrical wires).

We always keep old-fashioned mouse traps baited with peanut butter in strategic areas in the basement, to alert us to any potential future issues. But to date, since further sealing the basement and using the LED lights in remote locations, we have not caught or seen evidence of any further mice.

Hope you enjoy a mouse free zone soon!

Not an answer to your problems, but we had mice a while back, so I started setting out a mouse trap in the basement.  Since the basement is right under our bed, if the thing was tripped, I might be able to hear it.

Late one night, I heard it trip, but decided to wait until morning to dispose of the critter.  Went out the next morning and opened to basement door to find both mouse and trap gone.  I unloaded just about everything from the basement, only to find no sign of the mouse or trap.  Since I had left the sliding door to the underbelly open to allow them easy access to the trap, I figure the unlucky mouse wasn't killed, but dragged the trap back somewhere in the underbelly.

Still haven't found said mouse or trap.

Terry