I towed a tag trailer 450 km down the highway at 55-60 mph and got 37 liters/100 km. (6.4 mpg). Can you get a Dodge or GM to go through that much fuel?
I imagine you must have had some really bad head winds. Or even cross winds. The effective drag from cross winds is almost as bad as head winds!
Yes, that is crummy, and is certainly worse than I AVERAGE with my v10 pulling my monster trailer. But mine has dipped down that far on occasion, when the winds are so strong I can't hold OD.