That's an age old question that always opens up a can of worms.
Empty it out of anything that remotely smells like food and thoroughly clean everything, pantry shelf, fridge, stove, oven microwave, etc. Seal up every hole bigger than a pin head. Some people suggest steel wool but it's steel and will rust. I use stainless steel wool found right by the sponges at WalMart. There is a mouse proof spray foam you can use to fill all holes, gaps etc. Look under the camper where electric, plumbing, propane lines enter the camper. Waste lines too. Look inside your camper under the sinks where the water, waste line come into the camper. Remove the panel under your oven, if you have one, and check the LP line where it enters. Try to screen off the areas where your slide rams go into the camper.
Now the big question, what mouse deterrent to use. Ask 4 people and you'll get 5 opinions. There's Fresh Cab, ultrasonic plug in ones, dryer sheets, shaved up bars of Irish Spring soap, moth balls, etc, etc, etc. I can only tell you that we've had excellent luck with a combination of dryer sheets and shaved Irish Spring soap put into clean containers spread around the camper and in the garage areas. A package of Moth balls spread under the camper and around anything that touches the ground won't hurt either.