I have had several encounters with mice. Mice in the engine compartment is no big deal unless they have chewed on the wireing. Just be sure to brush out any nests before starting the MH.
Mice in the MH is a different story. I have filled any spaces in the floor with spray foam. I got under the MH and crawled around looking for places that a mouse could get in and sealed them. Pay attention to where the power cord comes in and close the space around it. I used sheet metal and duct tape.
Mice built a nest in my MH several years ago, they used insulation from the oven and anything else they could find, including dryer sheets. I now keep a loaded mouse trap in the MH and have been mouse free for several years. I think sealing any holes where mice could enter is key.