OK, so I took the furnace apart and discovered that two of the three burners had been plugged by dirt-dobber's nests. This prevented it from producing the heat required to trigger the fan to come on in a timely manner and when the fan did come on the heat was weak to say the least.
This furnace is a 1966 era model and has a cast iron burner manifold with brass burners. The whole assembly I removed weighed over 6 lbs not including the blower mind you! I has a pilot/safety shut-off valve, then a
regulator, then a solenoid valve, all separate.
The heat output is much better now! I don't know if it can keep the Travco warm in the coldest of weather, but it will in any weather I will be out there in. I know that this furnace is not as efficient as newer ones, but it is paid for at least!
6 hours ago