What I did on my deer camp camper is disconnect the original furnace and put in a single burner propane heater that mounts on the wall. I know this isn't the fix you were wanting to hear but it is by far the easiest.

I just crack a window to let a little air in while the heater is on. Never had a problem with carbon monoxide build up. I will not let the heater burn while I am sleeping but heat the trailer up before I go to bed. It will hold the heat fairly well and heats up quick again the next morning.

This is close to the unit I have in my camper. Works great.

http://www.ruralking.com/mr-heater-p...FeHm7AodmBUA5Q