That sounds about right. At 74mph I'm right at the edge of the green arc, about 1600rpm @ 6.8-7.3 mpg. I don't have as aerodynamic fairing over the cab as you do though.
You may be on to something with respect to the rear end gear ratio. I originally bought it with an 11,000 car hauler. I'll look into it. Also, what RPM is your engine running at 74 MPH?
I've got a Renegade 45' on a Columbia with a 450 Mercedes and only get 6-7mpg towing a Jeep at 74mph. What gives? Does that 4mph really affect mileage that much?