The V6 doesn't get that much better gas mileage on paper than the V8. A few trucks back I had a 4.6 litre F-150. It got worse fuel economy than the 5.4 litre version because it had to work harder to move the truck. Put a trailer on each truck and the 4.6 goes for ****.
Again, as I've said before, if you want fuel economy buying a full size lifted truck is not a good choice. If you are buying this kind of vehicle for the LCF (Look Cool Factor) than so be it but don't expect to save at the pump.
People who worry about fuel economy would pass out if you knew how many litres I put into a tractor trailer or an excavator after a day's work.
Also, the engine needs to break in before you should really be worried about fuel economy. I know my 5.0 litre 2014 FX-4 woke up and got much better milage after it hit around the 5000 km mark. You still are not going to see good fuel economy in a lifted full size truck because physics.