Poison works great. Use an anticoagulant (like warfarin) based poison. Just don't leave any accessible water in the bus. Most often, mice eat the poison then get real thirsty, leave the bus to find water, and die. We live in the middle of 13 acres of woods and the mice are constantly trying to occupy our spaces. My house, garage and bus are mouse free, although they have eaten wiring in my wife's car when left outside (some automotive wiring insulation is made of a soy-based plastic). That cost us about $450!
Here's what yas do!... You leave an opened Beer in the Bus. Next you put a small pile of gravel next to it. they come drink the beer and get so drunk they start throwing rocks at each other never to return. Just saying!... Dave5Cs