I have towed my 7700lbs 5th wheel for over 8000 miles with my EB and I tried all the combinations possible in term of speed, locking gears and even tried shifting manually.
I recently installed LT-E tires, run them at 70PSI and set the cruise at 60MPH. I get over 10MPG all the time. I have no turbo gauge so I don't know about PSI but this thing stays in 6th 95% of the time and this how I get the best mileage.
This is a super quite and efficient engine for towing. I'm so spoiled now that the only thing I am considering next is a diesel... nothing else unless ford comes out with a 5.0L EB next week... ;-)