I don't what Vince meant by coach battery. I check the voltage at the camper battery under load. Hook up the tow vehicle and camper electrically, turn on the fridge and turn on the running lights.
If the camper battery voltage is 13.0 or better when the tow vehicle is idling, that means the tow vehicle alternator is carrying the load, and trickle charging the camper battery. Take the tow vehicle engine up to 1500RPM, and if you get the camper battery up to 13.6V, you are good to go, and charging the camper battery.
The worst you want to see is 13.1V at the camper battery when at 1500 RPM. Anything less means the alternator is not carrying full load even at cruising RPM, and your camper battery is carrying part of the fridge load.
The reason for the full load test is that the wiring to the hitch on the tow vehicle is sometimes not heavy enough to carry the running lights plus the fridge without a significant voltage drop. That's also why you measure the voltage at the camper battery, and not the tow vehicle battery.
On my Coleman PUP fridge, I had a similar situation with the flame being outside the actual burner. The dealer found a spider web that was messing with the propane flow. Fridge worked great on propane after that. Given that the igniter and valves all work correctly, it's probably something simple like that.
Stuff I have learned the hard way
2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time