I wouldn't use poison, you don't want the mice going back to where they sleep (like your air ducts, in the walls, any hidden place) and dying. Set lots of traps with peanut butter and put LOTS of dryer sheets around the bus.
I am fighting some rodents right now, they are chewing up all the fabrics. I finally declaired ware on them a couple days ago and I hope to rid the bus completely of them.