I think aerodynamics needs a bit of discussion re stability, rather than being dismissed.
Aerodynamics is involved with our units, this is evidenced in the fact our caravans present “drag” which we feel when an oncoming lorry's bow wave slams the front. Also, it is more than evident in our fuel consumption when towing, often modulating as it reflects the headwind we encounter.
As the van's body frontal area is predominantly way above its axle this component of the drag will try to relieve the dynamic vertical loading on the nose. The “shadowing” and turbulence from the towing vehicle, unless that is Transit like, if anything will IMO lead to the centre of pressure being well up vertically, further decreasing the dynamic vertical nose load.
I suggest in part, this drag off setting of the dynamic vertical nose load, accounts for units with the higher static noseweights yielding more stable towing. The dynamic vertical nose load certainly needs IMO to remain positive.
This particular aerodynamic effect would IMO be little changed via setting up nose up or nose down.
I however, see potentially other air loading influences could be related to the van's longitudinal stance.
Whilst a box has little in common with the aerodynamic finesse of an aerofoil, the box's “incidence” to the air with its flat plate surfaces will still be there, both on the top and bottom. It should not be overlooked that whilst induced pressure changes might be small in magnitude they act over a very significant area.
On the top, roof surface, with the nose down the pressure point is IMO going to fall somewhat forward of half the roof's length, so add to the vertical dynamic nose load, offsetting part of the drag's affect.
Underneath, the turbulence from the front lower edge because of the incidence, will reduce the pressure on the immediate forward section of the floor, again adding to the dynamic nose load, and again offsetting the negative affects of the frontal drag.
Tip the van backwards so the floor's incidence creates a force pushing the nose further up, compliments the drag in together reducing the dynamic vertical nose load.