I have a D4 not a D3 but the manual says to NOT use a weight distribution system. The air suspension does what is needed. I also spoke to a sales guy at a van show selling WDH systems and he agreed with LR and using a WDH was a bad idea.
I did the full lap without one and the D4 pulled the van without issues.
One thing to look at is if you need an LED lights module. New vans mostly have led trailer lights and they dont draw enough power to let the cars computer know there is a trailer connected. I know the D4 has some smarts for towing but they need to recognize you are towing but the lights.(i am no expert, but i think it is worth considering).

