You need to check the DC voltage at the furnace when the APU is running. Usually when you have this type problem, the DC current is high(over 14 volts) and this causes the furnace fan to have higher RPM's and this causes the burner to have too much air flow and blows out the flame or never combusts at all due to the faster air flow. When you attempt to start, go outside during the 3 try attempt and see if you smell LP out of the furnace exhaust vent. If so, then the air volume is off due to higher fan speeds. Some Converters when they have high AC voltage(over 130 volts) will produce higher DC voltage. So, check the APU voltage to the RV. Doug
