This is a fairly easy hypothsis to test. Begin recoding a daily log of your MPG. After a couple of tanks, you'll be able to document the trend.
I top off my tank twice a week, Fridays and Sundays. This gives a way to see if there is a pattern in the partial tanks suggesting some MPG anomalie. The data suggests that if it exists, the period is about 200 miles or less. But the real answer is using a daily log to see if the problem(s) can be correlated to some specific and previously undocumented effect.
http://hiwaay.net/~bzwilson/prius/pri_tank.html
Bob Wilson