I went from a 1997 Ford Aerostar to the 2001 Ford F150 Supercrew with the 4.6 V8 towing a GVWR 3500 tent trailer. Mileage for the 3.0 engine in the Aerostar was 14 city and 21 highway, 16 towing.
Overall mileage with the Aerostar was 15.5 mpg. I kept records for both trucks on spreadsheets for my own use. I have a lead foot so mileage changed drastically when I took it easy. The F150 for the same trailer was 16 -17 towing and 22 highway and 13- 15 city. Overall mileage with the F150 was 15.3 mpg.
Moving up from the F150 to the F250 is quite a jump in truck size. The F150 weighs about 5300 lbs. and the F250 depending on what engine goes in can weigh up to 8000 lbs. The gas engine with a crew cab come in around 6900 lbs. while the diesel could be over 8000 lbs. on a crew cab. The Ford site has a Towing Guide http://www.ford.com/towingguides/
that would explain what each truck can do. The F250 will tow a much larger travel trailer and/or a 5th wheel. If you are planning on getting a new travel trailer or a 5th wheel then the F250 would be better. There is a site online called Changing Gears that has trailer and 5th wheel calculators http://changingears.com/index.shtml
that has a lot of info on what you are considering. I used this site when I bought my present diesel truck, and then matched it to the 5th wheel I wanted for towing. And as others have told you, towing is conditional on you style, weather conditions, how you maintain your truck and trailer.