If it is a motorhome and since you call it a coach I assume it is, the best defense is to make it impossible for them to get into the coach. The only points of entry where they can climb in are points that touch the ground. Tires, cords, hoses, etc. since they cannot jump any distance, if those areas are protected, they cannot get into the suspension, motor, etc, etc.
When ours is not in use but plugged in, I hang the cord up off the pad. You can also get cones to put on the cord similiar to what ships use. To protect the tires I park on aluminum flashing coated with car wax. It is too slippery for them to climb on the tires and gain access. Just cut the flashing a few inches bigger than the tire footprint.
I have also put a ring of traps around each tire which again denies access. Both methods work as we have not had any mice in several years since adopting these methods. If one you are so inclined, doing a combination of both methods would be double protection. I wipe the flashing off with a damp cloth before each parking to keep it slick.
We did also note that fewer mice got in when we installed the tire covers. I assume the vinyl cover blocked some, but not all. A few smart ones still climbed up the inside facing tire area.
We did try all the moth balls, dryer sheets, etc. all seemed to work some, but areas like the engine compartment, generator, etc are hard to protect that way and may require frequent changing to be effective. Not allowing them to get in seems to be the easiest.
You can also try urine from predatorpee.com sprinkle some around the tires and that will also help, but does require reapplication every few months or so. I use it to keep them off the deck at home around the BBQ area.
