An equal size/weight 5ver is going to have negligible mileage difference over a TT. If anything it might be a shade worse being taller or a-shade better if the nose is more aero.
If you’re getting 17mpg average on the highway repeatably, you’re doing as absolutely good as humanly possible with a truck that size. You probably don’t actually average that mpg.
Towing will get what it gets. Asking the question is like asking how many calories are in a pizza.....too many variables, crust, size, ingredients, toppings, did you eat the big slices or little ones, lol.
Same with mileage. Avg speed, acceleration rates, wind, hills, weight, aerodynamics, ambient temperature, etc etc etc.
One true statement is you’ll burn through a tank pretty quick. Trucks with these towing capacities, all of them, should come with big fuel tanks, imo. Thanks to Ford, wish others would follow suit.