I think a lot of people do not understand their Nivomat leveling rear suspensions.
You need to hook up trailer and drive, maybe just around your neighborhood. The shocks will jack up and level the truck. If you set your system beforehand this will happen and you'll find underloaded springs etc when you arrive.
Here is a quick 5 min video from ZF about them.
You basically want the truck to level it self, which a short drive will do. I wonder if you could just stand on the bumper / hitch area and "bounce" a little to pump the shocks up. Then apply the WD. I have the air suspension. Love it when hitching up etc, but I'm concerned with a system failure when towing.
But I would do some more research into the design of your truck first. The nivomat shocks are pretty unique.
With all due respect, just raising the rear of the vehicle with bags or a self leveling system does NOTHING for the safe handling of the tow vehicle. Solid blocks could be placed between the axles and the frame and all the weight will still be on the rear axle. A WDH must be used to mechanically transfer weight back to the front axle on a trailer that big. Anything less will be a very unpleasent white knuckle trip that will exhaust the driver IF he/she makes it to the destination.