Vehicles in sig:
In my 5.4L Gas Truck (3:73 rear end) I'd get 8 to maybe 10 towing (downhill with a tail wind). But it never did better than about 12.5->13 even unloaded and not even that good in cold weather.
My Current 2017 6.7 PD (according to the computer) got about 10.2 on a recent long weekend from Central Ohio, up to East Harbor State Park at Lake Erie, and back towing my camper 250ish miles in the fairly flat parts of Ohio. I think it may have been better had it not been for a mid-trip REGEN...
As my daily driver with a 3:31 rear end I get a calculated 19 MPG on my 65 or so mile daily round trip commute. (Which matches what the computer says I am surprised to say.)
The best MPG I've seen unloaded is about 21.5MPG (according to the computer) on a 240ish mile mostly rural highway trip I took a few weeks ago.
When the truck decides it's time to REGEN the MPG drops like a rock, towing or not.
I think that you will find that just about everything gets low MPG towing large trailers. There is a LOT of "sail" area back there you are dragging through the air and that's not gonna change because you switch out the motor... But that said, going from 8 to 10 for me... I'll take it.