Towing with the vehicle in my sig, on a trip from Harlingen, TX to Fairmont, WV. I averaged on the leg to Nashville, TN 12.8 MPG. The overall mileage to WV was 12.3 mpg.
The trailer scaled out at 8700 pounds and tow speed was 55 - 65 mph. The major difference between towing with a gasser is the ease that the diesels torque gets the load down the highway.
My Tundra pulling the same trip earlier averaged 7.5 Mph, quite a difference in mileage. The original difference in vehicle purchase price would help to offset the difference in mileage if the gasser was set up to handle the weight.
The Tundra on a good day empty would average 13+ MPH empty and the Diesel on a bad day 13.5 empty. The long and short of it is comparing apples to oranges. One is designed to haul a load and the other is a compromise. Compromise is good for the short haul and easier on the wallet.