My lifetime average is 15.3mpg which is combination of city, towing, and highway for 10,000 miles. Unloaded at 75-78mph I've averaged around 19-20mpg depending on wind. Around town if I dont hammer it I'll get around 14mpg, loaded with my utility trailer and mule pro itll do about 16mpg. My 14k fifth wheel runs about 10.5mpg which is on par with previous generation LML and L5P trucks I've had, and is better on mileage when compared to my 2017 super duty diesel.
Mileage wise I see an improvement on unloaded vs my 2019 L5P, other than that it is nearly identical.
However performance wise the 2020 is definitely smoother and seems to have more get up and go. I will be doing our yearly tow up to colorado from texas in June and will report back on mileage from that trip as our loaded weight will be in the 17000lb range.