1st off you shouldn't be using air bags if you're not needing them for extra gear in the truck.
However if you do haul a lot of gear in the truck then they work great.
From my past experiences using bags with a WD I found that the best way was to load the truck up without the TT, then air the bags to bring the truck to it's original height.
THEN hitch up the TT and snap the WD bars up. Loading the truck 1st and airing the bags to set the rear up will ensure that you can then transfer the right amount of weight to the trucks front after snapping up the WD bars.
Also if you're using a friction based WD then if hitching 1st and airing 2nd you reduce the friction.