It's all to do with power to weight ratio, the torque required to pull 38 to 40 tons; is far greater than that required to pull a Bentley ( which is about 2 tons ). The more an engine has to drive / work , the less economy it generates.  It has always been understood, that to run "Air Conditioning" in a car; will reduce its fuel economy and that's just having to drive the Air Conditioning pump, so to pull a Caravan....or tow 38 Tons of trailer...........well it speaks for itself. Attach your 5 litr Car to a medium size Horse Box and then see how good the fuel economy is.