I just towed with my '17 3.5EB quad cab long Bed. I had a 2013 quad cab with the standard bed. Both have the 20" wheels. Neither have the max tow. I tow a 30' Windjammer that I approximate at 8500#. There is over 1000# tongue weight. I towed to a familiar campground so I knew the road well. There was a large difference. I kept the same Husky Centerline weight distributing hitch. I immediately noticed that the trailer did not seem to wag me at all. There was just a small sensation with the 2013. With the longer wheel base, it really was very relaxing to not have that uneasy feeling. The extra power of the gen 2 was noticeable along with the 10 spd tranny. I also would average 10 mpg with the 2013 and was getting 15.6 with the 2017 (I get around 23.2 normal driving to work). I cannot make the statement that I didn't even feel it behind me but it was quite a difference. I have been very happy with this truck. There have been some minor issues but a local Ford dealer has resolved them all.
This is by no means a professional review, just what I have observed.

 

15.6 pulling a 30' 8500lb TT? You better check your LOM. Short of a 70mph tailwind, ain't no way.....

 

Did you hand calculate your MPG? I very much doubt this as well.

 

You are likely overloaded for both GVWR and RAWR with that trailer. And getting 15.6 MPG towing???? BS. I get 8-10 towing the same weight with an HDPP truck built for it.

 

I'd love to get 15+ mpg towing. I just towed my 8K car trailer this past weekend 360 miles RT, tail wind going, head wind returning and my avg. was 10.2. I'm very pleased with that considering.

 

Not rocket science. Miles driven by gallons put into take equals miles per gallon. I filled up before and after the towing. There weren't a lot of big hills and I wasn't throttle heavy. I was surprised too.

 

I towed 8500 , 1200 tong weight and averaged 11.9 . That was mainly cruising at 65 ish. No heavy throttle.

 

Yep, there's no freaking way that's accurate, period. I get that mileage on the highway at 85 mph, without a trailer or any cargo in the bed, and just my wife and myself (who don't even add up to 300 lbs) and a couple of suitcases. And my truck is running perfectly, with only 6,700 miles on the clock.

 

What were the raw numbers for mileage and gallons of fuel?

Could be as simple as a relatively short trip with not a lot of fuel and the variance in how 'full' the tank gets before the pump shuts off skewed your numbers

 

Assuming you have a high aero drag TT as well, I think 11.9 is pretty darn good for a trip average. No way OP's claims are possible for a trip average. Maybe for a short stretch 65mph flat no wind/tail wind, you might temporarily see 15, but it's just not possible for a trip of any length and normal conditions.