You should not be adding air bags/helper springs/etc to fix sag caused by your trailer. This is counter productive. If you hook up your hitch and then inflated your air bags, you just lifted the rear of the truck and released tension off the the WDH spring bars which is the whole point of the system. You might as well not use the WDH at that point.
If your weight distribution is set up right then the truck should be riding pretty close to level with the rear squatting a little and the front raised a little bit from unloaded. If it is squatting excessively in the back, then you either dont have your hitch set up right(or maybe you bought too light of a hitch since you think the tongue weight is only 750) or its simply too much trailer for the truck.
Air Bag/Helper Springs/ETC should only be used for weight that you put in the bed of the truck, and then only to level the truck back to stock ride height PRIOR to setting up the weight distributing hitch.
Trailer: 2012 JayFeather X213
TV: 2014 F150 Ecoboost - Lariat, 4x4, Screw, 6.5' Bed, Max Tow Package