Geometrically, I think the tongue weight should be slightly greater if the tongue is low assuming the center of gravity of the trailer is higher than the pivot point of its suspension, if there's no weight distributing hitch to mess with things. With a weight distributing hitch, I could see how the hitch geometry could easily come into play to alter how much weight gets distributed as its angle from horizontal changes.
If an air suspension setup weighs more than an extra leaf or helper spring, then it would load the rear axle more by that additional weight, trailer or no trailer. That part's kind of in the "duh" department. Putting a cinder block in the truck bed over the rear axle would likewise add to its weight in the same manner....
