If you want good mileage, dont keep buying Ford. Cummins gets the best mileage and duramax is still better then powerstroke in mpg. That being said, I love the Ford trucks based on how they look, and the 7.3 is a reliable beast. Diesel is better than gas when it comes to pulling often, fuel mileage, and engine longevity.
If you pull light and buy new trucks every 2-3 years then get a gasser. I am selling mine and will stick with diesel from now on. I get 13-15 pulling weight in city and have seen 27 mpg empty on a highway run. This is with a calibrated scanguage II which is very accurate. I have several mods on my truck which make a huge difference in mpg.
2005 Isuzu NPR turbo diesel 16' box
2000 Ram 2500 24 valve CTD
1996 Ram 2500 12 valve CTD
1999 F350 7.3 PSD flatbed
20' enclosed, 16' open, 14' dump
3 Walker MT's 48", Toro Z500 60",
Toro proline 48", Dixon ZTR 42"
Stihl small equipment
SC licensed pesticide applicator