I agree with most here. I have an ecoboost and at 100 KPH, the mileage is around 21 l/100km. At 115 KPH, it goes to 26 l/100km. My trailer is around 7500 lbs wet. I wouldn't tow heavier and I am at 3/4 of the rated capacity. It works well but any heavier, I would move up.
FYI, if you are towing, forget about gas mileage. You are towing a sail and that's just the way it is. After a certain speed, it doesn't make a **** bit of difference what truck is towing it, wind resistance will take over.