One of my buddies in Indiana has several RVs he keeps on his rural property. He swears by the moth balls to keep the mice away.
He keeps the moth balls in several plastic tubs with a lids. When the RV is being stored he takes the lid off of the tubs and places them inside the RV. When he takes the RV on a trip, he puts the lids on the tubs and then puts them in a storage compartment. He opens up the windows to air out the rig a couple of hours before heading out.
He claims he has never had a mouse in the 3 RVs since he began using the moth balls.