Check the burner.
One in our 89 GBMC had the same issue before. I ran down all the other options before discovering it was bad. Seemed strange that it worked for a while and then stoped, but it solved the problem.
You will have to unhook the propane line, remove the exhaust tube, unplug some wires and take out 5 (I think) screws to get the propane valve out with the burner attached to it.
Mine was blown apart, with a large hole in the middle.
New, it should look something like this:
Mike and Carla
1998 42' Overland Larado Diesel Pusher
On the road - here and there - Full Timing