With dual axle, hitch height is critical. If to low more weight on front axle and drawbar weight will be to low. Hitch up higher more weight on rear axle and heavy drawbar (safer of the two). So need to work out your best height, little higher if in doubt, plus need to take into account when fully loaded if your car sags, your van will be more unstable (as above). My 2300kg boat is dual axle and have played alot with this, not by choice.
Single axle, really does not matter, more to do with packing.
Single axle, really does not matter, more to do with packing.