I think you are underestimating the tongue weight. If those numbers are manufacturers advertised UVW numbers, they are notoriously wrong, and not wrong in your favor.
Another number notoriously wrong, is advertised curb weight. Anything added to the vehicle (dealer installed options / aftermarket accessories, undercoating, dirt, rust, etc) since it left the factory, added pounds to the curb weight and reduced available payload. Even something as simple as floor mats took about 15 lbs.
What you're trying to calculate, is so close to the edge, the slightest miscalculation, extra case of refreshments, unplanned passenger, or extra piece of firewood, will put you over. I think the underestimated tongue and curb weights are already going to do that.
I think you're putting too much faith in the WD hitch being properly set up and transferring the right amount of weight. Only way to be sure, would be to go to a scale, every trip.
You're putting yourself in a situation where you will need a scale at the door, to weigh every thing and everybody, going into both the tow vehicle and the trailer. You'll also need a tongue weight scale for loading the trailer to a specific tongue weight, everytime.
If the Expy has passenger "P" rated tires, you'll need to pump them up to max pressure, for towing. They were designed for comfort, not heavy loads.
Another area you may save a few pounds, is the WD hitch. Andersen makes a WD hitch with built in sway control, that only weighs 60 lbs, including brackets that are mounted to trailer tongue. On heavier trailers, it's a little weak on weight transfer, but, it works well with lighter units, and it weighs almost half that of many other hitches. An added benefit is, it's quieter than most hitches, and because it uses no dirt attracting lube of any kind, it is cleaner.
Do a search (Andersen hitch) here on the forum. It has been discussed many times.
