See my other thread on a big tow
Tundra as Tow-Mobile: 1,200 miles, 12,000 lbs, 10 MPG.
Finally, I did use the cruise control because otherwise without good foot control I kept speeding up to 70-75 MPH and watched my mileage drop. Used Tow/Haul mode which delayed upshifting and encouraged earlier downshifting (I think). I kept it about 60 most of the time. I used 91 Octane, mostly to stay away from Ethanol (yuck).
10 MPG wasn't bad for this big a$$ trailer.