Check out the Furnace Troubleshooting article in the forum Library. See http://www.rvforum.net/miscfiles/Furnace_Trouble-2.pdf
One guess is that the gasket around the exhaust and air intake has broken. That messes up the air/fuel ratio in the burner (causing smoke) and also allows the exhaust to enter the coach interior. The furnace has two fans on one motor shaft and they are separated by a gasket that isolates the interior (circulation hot air) from the burner intake & exhaust. If that gasket breaks down, the two air streams get mixed and cause problems.
The other likely cause is a hole in the burner heat exchanger, which lets combustion by-products from the burner get into the air that circulates inside the RV. They are normally separated by a metal wall and the inside air gets heated by contact with the hot metal. If a hole burns through the metal, the burning gas fumes get circulated inside the RV, and the burner smokes because it gets too much air.
