Mileage loss when towing

[GORDON HUMMEL]

I am 2700 miles into my spring trip & towing a car for the first time. I expected to notice some loss in mileage because of the added weight, but I was surprised at the results! The Newell weights 41250 & the car 3000 lbs, a 7% increase. My mileage previously had been 6.7 MPG over the last 30,000 miles. Towing, it has gone down to 6.0, a 10% loss.
For this trip I reduced my speed from 67 MPH, 1800 RPM to 62 MPH, 1700 RPM.
Also just install H rated steer tires inflated to 120 PSI vs the old tires @ 105 PSI. Figured this might provide some less rolling resistance ?
I try to drive as much as possible using the boost gauge as an indicator. In the past, not towing, I would run < 7 lbs of boost at 67 MPH. I am now over 7 @ 62MPH.
My question is am I expecting too much?
Am I running below the sweet spot in the fuel curve? Can't find one for this engine!
If I was towing 30,000 lbs, would my mileage be 2MPG ?

Any thoughts