Here is something to consider. I left the cold climes of sub-zero Indiana and headed for not so cold Louisville. No trailer just my 2011 F-150 with 15,000 miles on it.
Leaving in sub-zero temps with a stout cross wind I got 16 mpg. South of Indy the winds died and the temp was about 10 above and I was getting 18 mpg. Coming home a week later at 40 degrees and no wind I was up to 21 mpg, same roads, same cruise control speed, just better weather.
I mentioned this to the Ford dealer mechanic and he said the computer is set to run it rich in the cold weather. I say about 3 mpg richer.;)
My point is there are many variables that effect gas mileage. My limited experience with my new trailer indicates about 10 mpg which seems to be the norm for a decently matched truck and trailer, regardless of truck or engine type.
Now, I am bracing for the day we are towing and catch a stout cross wind in cold weather, :E (hopefully while headed south.)