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!...
Dave and Gordie I really hate to burst yer bubbles BUT........
Mice don't DRINK liquid at all! They get ALL the moisture their bodies require from the food they consume! (I know it doesn't sound right.)
But I have a b-i-l that has been a health inspector for yrs and part of his duties are rodent control.
He has been schooled on this and has corrected me more than once on it.
He said what the poison does is thins out their blood and they bleed out internally. Additionally he says most times they crawl into the smallest and most inconvenient spot around and die then the smell of them decomposing drives you nuts for ever. (until the smell goes away)
Now just so there is no misunderstanding he did say RATS do drink water/liquid and even though the poison thins out their blood as well. They do tend to go in search of liquid and die.
So all that said it seems a logical solution would be to get a couple rats to kill the mice and THEN poison the RATS so they will leave!