Those are pretty good mileage figures for a V10 running empty. The towing mileages isn't surprising.
I recently did a trip with both of my F350's running together.
-'97, 7.3 diesel E4OD auto/od, 4wd dually with 4.10's and 235 tires.
-'92, 351 gas C6 auto/no od, 4wd srw with 4.56's and 285 tires.
The diesel truck was loaded and towing, 20,000 lbs combined gvw.
The gas truck was running bobtail with minor weight in the bed, 7500 lbs gvw.
Both trucks ran together, same speed and route the whole trip. I drove the diesel and paced my son, who followed behind me in the gas truck. I drove at what I know to be the optimum freeway mileage speed for the gas truck: 55 mph.
Both trucks have dual 20 gallon tanks. Both trucks started the trip with 2 full tanks on each truck. We stopped together at the same gas stations on the way, and at the destination. I topped off the tanks on both trucks each time.
I found I was putting right about the same amount of gas in the one truck as I was diesel in the other one, each time. Amazing how they can get roughly equal gas/fuel mileage, all other factors being equal (speed, hills, wind, driving style, etc), when one was moving almost 3 times the weight that the other was moving!
Diesel really is awesome :B