Most important item is to make sure all food and food residue is removed from the camper before storage. Clean everything.
Set traps inside and check regularly. If you are going to use poison I would not put it inside the trailer. The problem with poison inside the trailer is it attracts mice! It smells good to them so they will eat it. Why would you put something that they like in your camper so they come in to get it? Attract them outside to the poison. Use a zip tie to attach the box of poison to the spring shackle so they get to it before they get inside.