Been watching this thread for a while, and even put in my 2 cents a while back. Reading this morning and it seems like there is nothing wrong with the heater and nothing wrong with the plumbing for the propane, and "probably not" the ducting.
(Please excuse the use of the word plumbing there)
So as I set here, it occurs to me that the water heater also uses propane, and could be using more than normal. Water heaters use two thermodiscs, one to control the water temperature, and the second, redundant one to act as a safety. I've had personal experience with mine where the water tank had vibrated away from the two thermodiscs to the point that neither were regulating the water temp. I realized this when I took a shower and the water was REALLY hot.
The problem for me at least, was the crappy factory installation which was falling apart, allowing the tank to fall slightly backward, and the foam sheet that holds the thermodiscs tight against the tank, had a small gap instead of actually touching tank surface.
I never thought of propane usage, I just wanted to get my water back into normal temp range. Also, water heaters burners occasionally get out of tune and burn yellow instead of blue.
Just a suggestion, but it might be worth checking the "other" propane user. Check for blue flame, correct water temperature, and a mud dauber home in the chimney.
Just a thought that may be way off, but won't take long to verify.
(That will be 2 cents please.)