I just got back from a 4,000 mile round trip from MN to Grand Canyon, with stops along the way. Out I-70 through the Rockies, back I-80 through WY, SD. I had cruise control on for the duration and got 9.5mpg on average pulling 5,000lb TT. I cruised at 70mph when the speed limit was that or higher. I drive a 2017 extended. One thing I learned from renting the 2018-2019 expeditions watching the turbo is actually locking out gears gets better mpg (the 2018-19 have a turbo gauge). This turbo engine is incredible (both gen 1 in the '15-2017, and gen 2 in 18-19). For most of the time my '17 expy was happy pulling 70mph in 6th gear. What I found though is it winds the turbos more. I locked out 6th gear and it pulled in 5th all day and actually nets out better fuel economy because the turbos are not running as high as they would be in 6th gear. I tried premium fuel like others have suggested but really didn't see any change. The biggest change was locking out 6th gear.
It was by far the most enjoyable towing experience I've ever had. I love that Ford gives me the option to control my transmission gears, lock out gears when in automatic, shift myself in manual mode, and the tow/haul with the engine braking is wonderful. The only disappointment is that there is nothing, even the Rocky's that gave this Expedition a real test. It just cruised up everything like nothing was happening behind me.