The fact is the sail area presented to the wind by the ‘trailer’ can’t act on the prime mover in the way that it does in the case of the sail area of a semi trailer acting on a tractor unit.Whatever happens to a semi trailer the unit is going with it but not a drawbar outfit.
As shown in the NZ photos.Agreed it might be possible to reverse the situation given an empty prime mover v loaded trailer.But either way it’s not a combined wind force across the length of the trailer and prime mover.Not so much an air gap as a sail area and wind force gap and divider.
None of which stops the wagon being blown over and dragging the trailer down with it, or leaving it stranded still upright on the road - or vice versa.