If that model has a heated underbelly, which it most likely does, check your furnace for a smaller duct (2") that runs down. That is probably the access point they used to get into the duct work. Plug it off with some 1/4" hardware cloth. It would be advisable to just replace all of the flexible duct work if possible as it is futile to try to cover up the heated mouse pee smell. Take some time and vacuum out the air plenum in the furnace as well. Ask me how i know all this :(. Oh, and the access to the furnace will be thru the return air grill under the fridge on the inside of the camper.