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?
Was the fuel you were running 'blended diesel' as opposed to straight #2 ? You will always get less MPGs anytime you are running #1 diesel (less BTUs in #1).