The OP now has a 03 HR. The older the unit the more chances there are that it's been run too far out of level creating a blockage, but the OP cannot answer that as he just bought it.
OP, is this problem on both AC and propane? If just AC one of the heating element (there are two) may be burned out. If just on propane then rust on the burner or a clogged flue could be the problem.
If it gets down to needing a new cooling unit you need to seriously look at going residential. I made the mistake of spending way too much money on my 1200 before finally going with a home refer. We boondock often and I thought the electric power needed would be a problem...NOT my 21 cuft Whirlpool draws less than a 100W light bulb and it runs about half the time or less. You can find my posts by searching DIY and Ivylog.
Way to go Usersmanual, you looked at his previous post (03 HR) and you did not say it was a dumb idea to replace the thermistor or maybe you decided to edit that out.:B