Dano,
Doug is correct. I had exactly the same problem as you. I worked fine on AC but when I ran on gas, I had warm temps (above 40-45 degrees).
It turns out my baffle had rusted out. I removed the baffle - and found the rusty part. Ordered a replacement, cleaned out the burner tube like others suggested, and fired it backed up. It worked just fine.
Doug (Please correct me if I'm wrong) - but I believe the heat source on AC/DC is against the boiler and it's a direct heat transfer whereas the Propane the heat rises, transfers the heat to the boiler "indirectly" hence the importance of the baffle to transfer the heat to the boiler.
Gas pressure is also very important, and my pressure regulator went out on my current RV and I had the same problem (I think I had some bad gas) had the tank inspected and cleaned and replaced the regulator and I've been golden ever since; However, you'd have other issues if the regulator wasn't putting out 11.5". Low stove flame height when all burners are on or the heater cycling when it shouldn't...