Wow.
I have only towed once, an 18' enclosed trailer. Had 2 dirt bikes, gear, tools, and a generator.
Only averaged 9MPG.
All around I get 24 except towing.
I only had about 2500 miles on it when we went biking.
Still only have about 7k, so am not sure I even have it broke in yet.
The trailer is 3200 empty, so figure around 4500lbs with gear.
My F150 couldn't even carry it, but with the air bags and load leveler the ED1500 sits perfect.
I only use Chevron Diesel, and am not a total lead foot, but haven't been able to get better than 26 open road empty.
I'm sure the Seattle Area doesn't help, lots of hills.
I did have to drive over one of the mountain passes with the trailer, but still thought it'd do better than 9.
Nonetheless, I love every minute in the truck, glad I took all the options, love getting spoiled.
Did you do all your towing on flat?