BTW, I did find on the Heartland owners website a warning about the operation the furnace at high altitudes...
Some owners have experienced problems with furnace operations at high altitude, where the furnace works correctly at lower altitudes. Propane suppliers in low altitude areas (less than 1,000 feet above sea level) may mix additives into the propane. While this can provide improved operation at lower altitudes, at higher altitudes (greater than 5,000 feet above sea level), it can lead to furnace failure, including intermittent lockouts and other symptoms.
If planning a cold weather trip to a location greater than 5,000 feet above sea level, filling tanks after arrival may provide for more reliable operation of the furnace."
So it seems this is a known issue to some manufacturers.
2018 Jayco 28RLS
2012 F-250 6.2L 3.73