Darrell is right. We did exactly as he described over Christmas traveling 2500 miles mostly on I-10 from Florida to San Antonio and back, pulling our Ford Fusion with NO generator or cab A/C use, going 62-65MPH (except for the last 500 miles which we ran at 70), and averaged 7.45 MPH.
This was slightly better than my average of 7-7.2. Shows how much AC and/or generator use impacts.
What surprised me the most was averaging 7.1 MPH when averaging around 70 MPH on the final leg.
Btw, I use the website
Fuelly - Track and Compare your MPG to track all this for the sport of it. Just enter your fill ups and the website does all the calculating for you.