Mice in RV living quarters

We have a 26' Sunseeker RV. When winterizing it last Fall I removed all food, paper products and extra clothing. This past spring I discovered evidence of mice, including an area they chewed in the mattress cover I had left on the bed (the only piece of cloth that remained over winter to protect the mattress). We had put dryer sheets everywhere and a couple of pouches of something that was supposed to repel mice, to no avail. I cleaned out the living area thoroughly then we discovered that they had nested in the "garage" area - the big storage area at the back of the RV under the bed. That area was full of mice droppings.
I did a search for "mice" here and only found advice for how to keep them out of the engine compartment - AFAIK they don't go there, just the living area. What can I use this year to keep the little buggers out? I don't like using poisons as that often adversely affects wildlife. TIA!