Firstly, do NOT use mothballs. Camphor (the chemical in them) is highly toxic to all living things. I use scented dryer sheets, placed in every cabinet and under the drawers. Supposedly, they are not harmful. I hope not. The "packs" mentioned may be of the same ingredient... I don't know. I have never had a rodent problem, so i don't know if the sheets work or not. lol You should [of course] search for openings. You can also have your rig leak tested. The test consists of filling the unit with a non-toxic, non staining, odor-free "smoke" and pressurizing it to look for smoke escaping. It is usually done to look for potential areas of air and water leaks. If you have access to a "manhole" type (confined-space) [fresh air] blower, you can purchase the "smoke" from any HVAC supplier and do the test yourself. You only need a sheet-metal duct adapter to go from the round tube to the square roof vent.