You can have torque and HP and still get good mileage, which is why the 505HP corvette can still pull off 25-28 MPG highway. The main thing that decides mileage is the aerodynamics of the truck - during highway driving. For city driving, its how much the vehicle weighs.

Theoretically, a 500HP and a 250HP engine in the same truck, assuming both engines weighed the same, should get the same mileage, as long as they are both accelerating, traveling, etc.. at the same speed