Heck I get less than 7 MPG towing a 30' TT but that isn't bad considering it's mountainous here and that costs fuel. In the past the manufacturers used to put higher number rear ends just for the mountains and elevation. Regular gas here is 85 octane but Ford says don't use it in the EB. Agree with others that switching ratios probably wont payback the cost. Just something to remember when you buy a truck next time.
For those saying slow down I also agree but life is too short to go 55MPH when the towns are hours apart.

 

A very good thought. Make the same run in 5th and compare.

 

Not sure how it compares but my 5.0 towing my GN horse trailer with the loaded weight of 7 to 8 k gets around 10 at 65 to 70mph. My gear set is 3.55 and I often lock out 6th if we are in the hills of KY and WV. I think my wind drag is probably a lot less than yours though.

 

I almost always lock out 6th gear. I generally use 5th and often 4th. Thanks for the advice though.

 

I pull a 33' Springdale TT loaded around 8k. My 14 XLT 4x4 Off Road Boost has the 3:73 gear w/ HD Tow Package. I set the cruise on 60-65mph....and leave it. The truck just seems to pick a gear and hold it...never hunts. She pulls those 6% grades stress free. I average around 10-12mpg. Coming from a Titan.....well its nothing short of amazing. It flat-out kicks that Titan to the curb!