So I had a reg cab dually gasser from Oct 2019 to Dec 2020. I logged 16324 miles. My average economy was 13.0. it was 459 engine hours, 79.6 idle engine hours. I hauled two different gooseneck trailers, 547 miles for one, 761 miles for the other. Avg economy was 8.2 mpg. This trailer was loaded with 14 large round hay bales for half that mileage, empty the other half. Country roads, with some stop signs and traffic lights. I towed my travel trailer, averaging 8 mpg. A friends travel trailer only got 7.3. My average with my aluminum stock trailer was 10.8. I used 4% of my front brake life, 3% of the rear. I have pictures of all this data. I had measured with manual calculations many times, and the computer was very close.
If I was planning to keep my truck on order, I would have thought more about the gas engine. Since I am planning to flip, diesel was my decision.
I used a 2019 crew cab dually diesel to take my trailer to Kansas in July 2019, and averaged 9.9 mpg. I used my Express with the 6.0 gas to take my trailer to Iowa, and averaged 9. (I found an air deflector the day before departing, and driving 62 mph, I achieved the 9 mpg).
Overall, I have been satisfied with my gas engines.