Hi,
I have a small heater that is 150 watts and has a magnet to secure it to the propane tank. I have not had to use it yet. It can be used on my generator as well to act as a block heater.
You may have a combination of problems. It may not be 100% propane. Condensation may have formed over the years.
As the volume of liquid propane in the tank drops it is easier for the entire amount to get down to where it does not boil well.
Both the furnace and generator want a lot of fuel. The temperature on the tank may be well below the ambient.
Magnetic Heater
