South Dakota to New York, strikes me as a significant amount of relatively flat, and downhill (Prairies to the ocean) highways that worked in your favour. But that is good for towing another vehicle nonetheless.
Weight is certainly a factor when towing, but my personal opinion is that the aerodynamic profile is a much greater factor. I have towed a 4,000 lbs. (loaded) "flat bed" trailer that rides lower than the tailgate and gotten 16 MPG. When I tow my 3,500 lbs camper which is 10' tall that mileage drops to 10-10.5 MPG!
That little car on a dolly is "drafting" behind the Ridge. My camper trailer is like pulling a parachute!