For those that leave yours open... I only left mine open when not in storage. One day, I found the fan didn't work. I assumed it was a fuse or the switch. Nope, after removing everything, I found a mud dauber nest on the fan. When I purchased the camper the dealer tried selling me those screens. I bought one for the furnace exhaust but not one for the vent fan outlet. It must have caused the fan to burn out as after removing the mud, the fan still didn't work. Had to be replaced.
TT: 2012 Grey Wolf 25MB
Other Ride: 2006 Harley Ultra Classic