You can go to a local hardware store and get a mouse/mole smoke bomb. The burn hot so you need to be careful. Put it into a large cooking put, make sure there is no flammable liquids in the bin, make sure there are no flammable materials near the pot, light it up, close the bin doors and wait.
If you have celling fans in the camper turn them on to pull the smoke up. Mice and snakes will move in the opposite direction of the smoke.
You can use these under the camper periodically too. Again don't use them near the gas tank, you have to be smart about it.
I do this under my camper a few times a year. I also keep mothballs in the bins. So far no mice and no snakes.