I believe I posted something on here about the use of mothballs so I won't go into that again. I do want to inform some of you on the use of rodenticides. First of all there is no secondary poisoning. If a mouse eats the bait, and a cat eats the mouse, there's no effect. A 10-15 pound cat would have to eat about 22 mice at once before it is recommended to have it checked by a vet. A simple dose of vitamin k reverses the effects. Rodenticides are anticoagulants. They kill by internal hemorrhage. The reason why 12 different dcon products are being pulled from store shelves is because they are being used incorrectly. Don't just throw the rodenticides under your trailer and expect only mice to eat it. You can buy or build a refillable device to store the bait in. Use block type not pellets.
I would recommend a baiting program outside with snap traps inside. Put bait stations near the tires outside. Check and refill every 2 weeks. Install snap traps in all the dark places inside. If you can't check them on a regular basis, put a piece of cardboard under the trap to prevent the dead mouse from sticking to the floor. I know gross, its what happens when they are not removed right away.
Exclusion is the next step. Try to seal up all the entrance holes. A big one I found is the shore power cord entrance. If the cord is being used it hangs down giving mice a bridge right up to the trailer/motorhome. Try to use a copper or steel wool material, coarse not fine. Spray foam will eventually get chewed through.
To read kurv's post on the use of mothballs, GO HERE