Yes it is due to better aerodynamics. My 15000 lbs trailer is easier on fuel to tow then my loaded single axle service trailer that is registered for 3500 lbs. Also I towed my friends 7500lbs GVW TT and my fuel consulting gauge showed much lower MPG at over 40 MPH speed. In a hill is a different story as the weight will effect the fuel consumption.
Barbara and Laurent, Hartland Big Country 3500RL. 39 ft long and 15500 GVW.
2005 Ford F250 SD, XL F250 4x4, Long Box, 6.0L Diesel, 6 Speed Stick, Hypertech Max Energy for Fuel mileage of 21 MPusG empty, 12.6 MPusG pulling the BC. ScangaugeII for display..