Sulfur

• This bears mention before anything else, since it can easily indicate a life or death problem in your RV's heating system. Liquid propane and natural gas are naturally colorless, odorless hydrocarbons, lacking the unstable organic molecules that vaporize at room temperature and give other hydrocarbon fuels that signature solvent smell. To meet federal safety standards, propane retailers must introduce a bit of sulfur gas so that leaks are detectable. A little bit of the rotten-egg smell is normal when you first trigger the pilot light, but a continued sulfur stench may indicate a leak in the system.
  
  

Normal Dust

• Dust is a complex substance, composed of many different elements that combine to make up a fine powder. Some of these elements, like airborne silica, are incombustible or won't combust at the furnace element's running temperature. However, most household dust comes from either manmade fibers or organic sources like hair, pollen and the dead skin cells shed by occupants in the vehicle. While the RV furnace intake undoubtedly has a filter, dust may still infiltrate by going through small holes in the filter or coming down through the furnace exhaust.
  
  

Road Dust

• RVs face a particular challenge in the form of "road dust," which makes up about 30 percent of air pollution in rural areas. Road dust is a combination of brake dust, primarily wood-like organics, resin and carbon, aerosolized asphalt tar, tire rubber and particulate emissions from diesel engines. All of these can infiltrate an RV's furnace system and land on the elements, emitting any number of noxious fumes and odors. Road dust is a foreign odor for those who haven't spent a great deal of time on the road, but it's almost inevitable in some form or another. Reduce the road dust smell by installing a filter behind your furnace's exhaust and heating vents, and by keeping the intake filter clean and fresh.
  
  

Mold, Mildew and Other Icky Things

• An RV furnace is an ideal home for all sorts of organics, at least for those months of the year when it isn't several hundred degrees inside. Even when the furnace is on, mold spores and bacteria may survive harsh winter months by hanging out in the not-so-hot areas of the enclosure. Mold and mildew don't smell like mold and mildew when they burn; if you've ever smelled roasted or burnt mushrooms then you'll have some idea of the odor involved. Larger organisms will often make their homes in dormant furnaces. Some bugs and organisms will cook when you fire the furnace up, but even the smallest of them will leave waste products in the housing. When the furnace fires up, suddenly high temperatures will vaporize the more unstable organic molecules and emit a stench of ammonia, death and other unfortunate accoutrements of former life.