Moth balls. I used to buy a box every fall for my pop-up. I would put about 5 mothballs in each dixie cup and put dixie cups in the drawers, storage compartments, etc. Basically everywhere insde my pop-up. Even left a few sitting on the floor. On the exterior, I would put a cup behind each exterior access panel (such as the water heater) and even set a couple in the battery/propane compartment. Basically, anywhere I could fit a cup.
In the spring I would open it up and collect all the dixie cups, then leave it open for a few days. Never, ever had a problem.