I had a 08 6.4 truck. Fuel milage sucked compared to my 03 7.3 with full Banks kit. When diesel got to be $1 more a gallon than gas and I discovered I made a mistake buying a Supercab instead of a Crew cab I ordered a new 08 V-10. Another gas King Ranch. Only way to get this combo is order it. Was not towing much anymore. Gas truch gets 1 to 2 MPG less than the 6.4 did. Just bought a new 27 ft travel trailer. V10 pulls it OK but fuel mileage goes to hell. 7 to 8 MPG. Now I do tow 75MPH. V10 does not pull anywhere near as well as diesel but I am OK with it. Overall I like the gas truck better and wont lose any sleep worrying about owning it when the warranty is gone. Did not want to own 6.4 without warranty.
Have had large class A motorhomes with gas and diesel engines and would not even consider one with a gas engine again. But our pickups dont have big cats and cummins in them.
BTW 6.4 was faster than V10 in all catagories.