We just traded in our 2012 Mega Cab 5.7 with 26,500 on it. It was strictly for towing our trailer or say weekend trips. Other than that it pretty much sat in our garage.
When we first bought it around the city driving was around 9mpg. Highway I could get 11-12 out of it.
Towing the travel trailer at 8500lbs was around 7-8.
After the first year I put a cold air intake on a Flow Master exhaust and then added the bully dog system and tuned it for towing. Did much better and as time went on the mileage starting going up.
We drove just the truck to Maine and was about to get 18 mpg out of it.
We stuck our 33' ATV trailer on with 2 machines and a full 40 gallon gas tank on the trailer and got 16 to upper New Hampshire.
We traded our TT for a 11,000lb 5th wheel our last trip with the truck was to mid Pennsylvania and was able to pull 11-12 out of it between 60-65 mph.
The way home I was able to get 10-11mpg.
I know it's not the engine you were asking about but giving you some information that I do have.
We recently upgraded to the Cummins and a 3500.
Hope it helps you out some.