I suppose fuel could have been lost to the EVAP system, but an entire gallon per fill seems like an awful lot to lose that way. Maybe your alignment is off? That can be the case even if a car feels like it's OK. Have you double checked the tire pressure? One could have a slow leak that's dragging you down.

My ScanGauge originally did the opposite, reporting 6-10% below actual fuel efficiency. Adjusting the correction factor on the "Fillup" screen based on my receipt has given me relatively accurate readings ever since. That's the easy part of your problem to solve.

If I'm not mistaken, Flagstaff is WAAY above New Orleans' elevation. Is the 19mpg a one-way figure or the round trip?