Most any gas truck will average 8-10 mpg towing an RV of any size. I think you will be in this range, especially using non ethanol premium. If you can get 10 mpg towing in flat country, that will be better than most.
Since the beginning of the Ecoboost, the sales folks hyped up the terrific mileage, that upset many owners, when it didn't happen. Many used the available power, always had a heavy foot, and others towed heavy, at speed, and the mpg hoped for never showed up.
Yours is new, get a few miles on it, get to know it better, find what fuel works best, hope your experience with it improves!
My F250 with 6.2 gets better mpg using pure premium, towing or not.
Jerry