[Wavery]

It's most likely an air flow issue. The furnace has a "Sail switch" in the air intake and the exhaust outlet. If there is any airflow disturbance when the fan kicks off, the gas valve will shut off immediately.

It could be spider webs, mud dobber nests or a host of other obstructions. Those furnaces are pretty indestructible and very serviceable. They seldom need replacing.

Having said all that, we never use ours. They are so noisy and they put out so mush heat that they fire-up, warm the space quickly causing the thermostat to turn it off. 5-minutes later, the camper has cooled down and the thermostat turns it back on (about the time that we fall asleep). so it goes all night long.

We bought a Mr Buddy heater. It's silent and does an amazing job of heating the camper. We've used ours in +15* temps.