Before I gave up on it, I would take that flu apart and look for obstruction. In over twenty years, lots of things could have nested in there or soot could be evident. I would clean it up real good in the heat stack, check the flame carefully, and turn it on gas and leave it on at max setting for a week and see what happens. Make sure the coach is level when you do this.
It may be just a gone frig, but I would sure try running it a long time and then running several cycles of off and on for two or three days after fully cleaning and inspecting before I gave up on it.
I had one years ago that was like what you described. It was my first RV, a C, and I didn't know come here from sic 'em. A local rv dealer took it completely out, inverted it and two guys shook the heck out of it in the upside down position. They called it "burping the reefer". It worked perfectly for all three years we had it. Apparently, the gas had settled someway. All I know is it seemed to fix it.