I have just made two trips, one of 1,200 without a trailer and one pulling my motorcycle and trailer.
One trip was from home, Mountain Home, Arkansas to Pensacola, Fla. (1200 miles round trip) According to the car's computer I got 28 miles pre gallon. Mostly 4 lane highway at speeds of 65-75 miles pre hour.
As I live with disabilities the second trip was from Home to the Chicago/Rockford area. I was pulling a motorcycle (Yamaha Venture , close to 900 pounds) and a trailer (600 pounds) my total gas mileage was around 21 mpg. Again , mostly highway at 65-75 mph.
I have a 2008 C70 convertable with a manual 6 speed trans and burned regular gas.

 

That's about a 25% drop. I think you would have done better keeping it around 65 mph or less. Also, roads to the north are a lot hillyier than going towards FL. That likely had some effect as well.

I was moving with my wagon a couple of weeks ago and I only seen about a 8% drop with the wagon loaded down. Maybe 300 lbs per trip.