Sounds about right. My dad's '07 3500 Ram, 6.7L Cummins, 6 spd manual, 4X4, crew cab, long bed gets about 9 MPG towing his 40' travel trailer, but he doesn't mind keeping it around 75...sometimes 80 MPH on the interstate. He gets about 15 MPG unloaded. He thought it would do a little better once broken in, and maybe it will. He bought it new, and it still has less than 30K miles on it.