I actually weighed both tanks and subtracted the tare weight. Yesterday I refilled the tank that we used for 6 days, of which 3 nights were below freezing. Total used, 2.8 gallons.
The problem is probably not air in the line since most of the time it will light, but not on the first try (furnace tries 3 times). After filling the tanks, the furnace lit on the first try most of the time. Before filling, it was normally on the second or third try and with some failures.
I think I will replace the propane regulator and see if that has an effect. We have another trip planned for northern AZ later this month. That will be a test of the furnace for sure. I think I will try this regulator:
It has been mentioned as a replacement for the original equipment regulator.