You might try adding air to the air bags and then hook up the trailer, then adjust the rear height with the WDH bar tension. But you need to make sure you don't take too much weight off of the rear suspension to provide good traction.
Even my medium duty truck with a full air ride rear suspension that automatically adjusts for loads, sinks a little (about 3 inches)when I put the weight of the trailer on it.
Keep playing with the tension bar adjustment until you get the ride you want and then be happy with a little sag if it's there.
If you pay attention to the TTs going down the road, nearly half have a tow vehicle with rear end sag even with a WDH.