Furnace exhaust smell

As a fellow DIY guy, take this advice at you own risk. It's a heat exchanger, meaning that there are two completely separate and isolated heat chambers. The burning chamber intakes and vents outside, burns propane and exhausts carbon monoxide (among other things). The heater chamber intakes and exhausts air into the coach and should be clean air coming and going. Any leakage between the two can/will cause carbon monoxide to get into the coach (deadly-make sure your CO2 detector/alarm is working), smoke to get into the coach, etc. A early season "burning" smell in the coach is typical of the first use burning off dust in the system. Visible smoke in the outside system is indicative of incomplete burning which can be poor fuel/air adjustment, blockage (dirt dauber nests) in the heat exchanger, etc. Compressed air blown through the two systems every year can clean out minor stuff, or provide indication of more severe blockage. Mine can be removed for bench repair as a complete unit. Good to do every couple of years anyhow. I keep screens (camping supply house)over the outside intake/exhaust ports to discourage insects. Turn off gas, disconnect DC voltage, disconnect control lines, disconnect gas lines and remove unit. On the bench, disassemble, clean, inspect (especially the heat exchangers), lubricate, repair as necessary, reassemble and reinstall. I recommend replacing the whole unit if you find any problems with the heat exchanger, everything else can be replaced. Not difficult nor strenuous process, a half day's work unless major problems are found. Proceed with caution!