My RCSB 4x2 tradesman with the 3.6 and 3.21s gets pretty ridiculous mileage on the freeway. When I had the stock 17" wheels it would claim as much as 29.5mpg at 70mph. Slow it down to 55 and it jumped to as much as 33.5mpg. Long trips to the coast, flatland, mountains, hills, with some luggage averaged about 27mpg overall.
Town mileage has been around 18.0, and my commute, a mix, averages about 21.
I don't drive fast, and don't press the go pedal very hard.
It now has 20" wheels and mileage has dropped 1 or 2mpgs. Might be temporary as the tires are new and still breaking in.
The truck weighs 4500 pounds, confirmed on a scale.
As for if the 3.6 has enough power, I'm old enough to have driven the trucks of the 70s and 80s when "power" meant a 155 horsepower smog strangled V8 hooked to a sloppy 3 speed auto. Or 140 horsepower straight sixes, and even once had a GM 6.2l Detroit Diesel with all of 130 horsepower in a 3/4 ton 4x4. We towed, hauled and worked em', hard. They got terrible mileage, town or freeway, even that diesel was awful.