Looking for MPG (UPDATE)

I have a 2013 F250 crew cab 4x4 and get 19, 20 non towing and 10.5 to 11.5 towing a 5th wheel that weights 12400 at 65 mph. True about the 26 gal tank, I added a 30 gal auxiliary gravity flow tank in the truck bed. Works great and gives me enough fuel for a days travel.

I got a '12 6.7 and get between 16.4 - 20 mpg non towing. I'm really impressed with this trucks fuel mileage.

Here's a picture of a recent trip to our deer camp with 3 people and supplies in the bed.

Your post omits whether you're looking at a new 2015 model or a used vehicle.
One must also recognize that a dually won't achieve the same mpg as a single rear wheel machine.
The rear end gear ratio will also affect mileage.
Speed also greatly affects mileage.

My 2015 Ford 6.7 diesel CC dually will get 12.6 mpg loaded at 65 mph and only 10.6 mpg at 70 mph!!!

That said, the Ford 6.7 diesel is one quiet, powerful amazing machine. Enough so, mpg, while important, seems insignificant when driving it.

Have you checked out Fuelly.com? On this website, many owners are charting out their fuel economy and typically tag notes along with their fuelings estimating tows/weight/% of the tank towing. Strongly recomment hopping over there and looking over the year of Ford you are interested in looking at. I'm sure some 2015 owners have started posting.

I get 17-18 not towing on the highway. 10-12 towing depending on winds and grades. Your 6.7 is a newer version tho if you are getting a 2015. And mine is a crew cab with 4WD both of which reduce mileage a bit.