We had mice come in the trailer 2 springs ago, they left droppings and nut pieces all over the place. They even gnawed a whole about the size of a quarter through my canvas end! :M :M
What I found was several large holes under the floor that was not sealed from the factory, I ended up caulking the with silicone. Haven't had a problem since.
The spring when we had the problem, the previous fall had used loads of bounce sheets & the moth balls...didn't make a difference....
Mike