[JJaxon]

If it is the intake, blocking it off will cause your furnace to overheat and shut down. If you are wanting to block it off when not using the RV furnace, temporarily place a piece of plywood cut to size in front of it to block the mice trail, tape it in place, then remove when spring comes and the rodents go back outside. Mouse traps baited with peanut butter is an easy kill method if you need to be on the offensive.