I had a similar problem with the forced air furnace in our truck camper ('06 Okanagan). The fan would run and the flame would come on occasionally but not for long. After a good deal of investigation and detective work, we narrowed it down to the limit switch.
This switch is another fail-safe like the sail switch, but it monitors the furnace internal temperature. If it thinks it's too hot in there, it prevents the flame from firing.
A local RV repair shop quoted $450 to fix it, including re & re the furnace, disassembly, replacing the limit switch (and the sail switch too, being as how they already had it apart), and shop supplies. I decided I could almost buy a new furnace for that, so decided to have a go at fixing it myself.
Making a short story long, managed find both the limit switch and sail switch through the panel on the side of the camper. Had to removed much of the circuitry, the fan and the shroud that it turns in. This gave access to the two switches.
Found them both on Amazon.ca for approx. $25 bucks each. Replaced them both, and the furnace has run well ever since (almost two years now). It was the limit switch that had failed -- on the bench with the ohm meter, the removed sail switch worked perfectly.
Good luck with it...!
