If the rear of the car is hangin' low, all the changes to the hitch bar in the world aren't gonna make a difference. That adjustment is so that a vehicle that is ABLE to carry the load on level can be at varying heights - like a 1/2 ton truck often sits a little lower to the ground than a one ton - but either could carry the load without bottoming out the rear end.
Your situation is different. The hitch bar should be set so that IF the van sat level AND the trailer sat level it would connect the two. Will the air shocks help force things level - probably. I would still do the bathroom scale stuff outlined above to get the trailer properly loaded. But NONE OF THIS MAKES IT SAFE. Safe is making sure that when all of your stuff is loaded the GVR of trailer is not exceeded, the GVR and GCVW of the van is not exceeded, and that the rear axel of the van's GAWR is not exceeded. That's what will make things right. Not on a soap box, just trying to be clear. You see, if that rear end is sagging, that means you've got alot of weight on it that the design did not call for. That means the bearings in the axel are carrying more weight than they may have been designed to bear. Shoving air into a shock to force that rear end up does nothing about changing the axel's ability to bear that weight.
Worsed of all, IF you were to wreck, no matter who would be at fault and if you are over any of the measurements outlined above, guess who the courts would be interested in. See what I mean? Again, not trying to be awnery. Just want you to have all the info...