With air suspension the car will actively try to self level - that is the nature of a well designed air suspension system. It actively works on the suspension all the time, unlike a passive steel sprung system which reacts to forces applied to it and then settles.
The WDH is like a double lever, with a common point being the towball which acts as the end of two levers, the other ends of those two levers are towards the front of the car (for simplicity the front axle) and the van (the van axle)
By leverage when mass is applied to the towball, the force (weight) moves the ball downwards (and hence the front of the vehicle upwards - along with the rear of the van). The WDH when connected acts as a lever to transfer this force (weight) to both the front axle and van axle, effectively putting the force (weight) on them and removing some of the force (weight) from the ball and hence rear axle.
The WDH acts to limit the suspension travel when attached, and reduces the travel it would normally have. Given the air suspension is an active system that uses the air pressure and monitors to actively try to level the ride, the WDH can interfere not only with the self levelling but also introduce forces in opposition to the air suspensions normal operation, especially on bumpy or undulating roads. The dynamic forces introduced by the WDH will be increased when going down a dip as the vehicle bottoms out, and reduce or even be negative when cresting a rise, or so it has been described to me if I have understood it correctly.
I understand it is for the reasons of introducing forces that were not designed for is the reason why car manufacturers say not to use a WDH with their OEM air suspension systems. (Quite different approach may be needed where an aftermarket airbag helper system is applied to a conventionally sprung vehicle). As an aside hydraulically suspended vehicles (I refer here to Citroens) also do not have WDH recommended, indeed they recommend against them.
In terms of steel sprung vehicles, especially those where a separate ladder chassis is not included in the vehicle, many manufacturers also recommend against using WDH. This is for the reason that the WDH acting as a lever has a pivot point introduced somewhere under the vehicle (in order to transfer the force forward).
This will introduce stress and forces at a point where the body has not been designed to accommodate this, often leading to cracking and distortion, as the forces also tend to be in an opposite direction to gravity and again dynamic loads can quite quickly multiply the force (weight) applied.
Of course this will also apply to the air suspended versions of those same vehicles.
Another reason for also being wary of WDH's is that they tend to introduce further length behind the rear axle to the towball. The VW OEM towball mount would have the towball about 300-400mm closer to the rear axle than a say Hayman Reese WDH mount.
This can be important in terms of stability. A car and van act like a double pendulum with the ball being the second pivot point. Simply put the further out that second pivot point is from the axle (the primary pivot) the lower the speed at which sudden and uncontrollable instability may be reached. And the relationship is not linear as I understand it either. On my Touareg the ball is a bit under 1.00m from the centreline of the rear axle. By comparison Landcruisers with WDH's have the ball anywhere from 1.3 to 1.5 m or even more from the rear axle.
As all this is quite a bit off the original topic, for more on Caravan stability see some of the articles by Collyn Rivers who has written and researched this topic extensively. Google him but to get you started here is a link:
Caravan Dynamics | Caravan and Motorhome Books
Bottom line is I would not use a WDH with air suspension full stop. It introduces too many forces that are not factored into the design of such an active suspension.
And I don't trust insurance companies not to try to use the manufacturers recomendation to wriggle out of a payout should the worst occur.
I follow Rosco's advice, though our van is quite a bit lighter than yours.