I was reminded of this while reading Melissa's advice about black water tanks. I had both electric and propane heat in my rig but never used the propane. Good thing too. When my gennie was being fixed, the service guy noticed screening in the exhaust outlets for the propane furnace. He asked me about it and I said I thought the previous owner did it to keep out mud dobbers. He agreed it would work for that purpose but it would cause a problem if I used the furnace; like melting, or a fire. We removed the screening material and installed two little wire mesh baskets over the holes.
All vents should be checked and cleared of insects and their nests. Especially your fridge vent.