Have you been pushing the speed? When I commuted 100 miles R/T, the difference between 65 and 70+MPH accounted for 25 or more mile difference on a tank.
I do admit that I drive fast. The section of the GSP where I commute runs 75+ MPH on a daily basis. But that has been very consistent, with no changes in recent months that would explain the drop off.
If I drive the Double Nickel, aside from being a hazard to navigation, I would get about 300 miles per tank. But I only did that a couple of times as an experiment.