Answers to your questions David;
1. Truck Unloaded Weight measurements had the hitch on the truck, trailer disconnected.
2. Trailer weights were measured when I originally purchased the trailer with dealer configured WDH setup on my Tundra and it tracked great with no swaying. Latest estimated total weight of 5000 includes gear and full tanks. Initial trailer specs were UVW of 3910Lbs, Dry Hitch Wt. 450Lbs.
The key is stability is about the tow vehicle, not the trailer. That is easy to see. Fifth wheels and gooseneck trailers are considered stable. As are 4-bar linkages. What's the difference? The moment arm applied to the rear axle. I'm now going to ramble a bit to explain.
That is not to say all 5th wheels and goosenecks are without their own issues. On a short bed truck (anything less than 8') there is not enough room for the 5th wheel to rotate for low-speed maneuvering. The fix is either a slide hitch (multiple manufacturers, which can manually be slid about 14" toward the back of the box), autoslide (two manufacturers, automatically slides back if the truck turns), or a Reese sidewinder pin box (one manufacturer, the trailer rotates at the pin box mount, about 22" behind the coupler). Only the sidewinder works with Rambox. The last may increase stability since horizontal forces are applied to the truck on a 22" lever arm). Goosenecks, which appear to be the dominant choice for car haulers, have offset couplers which in effect extend the tongue. B&W has a 4" offset ball which does move the pivot and 10" offset coupler. Apparently it works with a 6'4" Rambox.
There seem to be two (three? four?) practices in setting up the WD bars. In one, the bars are adjusted until the front and rear heights are equal. In another, until the front is restored to its original height (all the net tongue weight on the rear axle). The OM says, for the air suspension, to only return 2/3rds of the weight transferred off the front wheels back. That means the rear wheels gain about 115% of the net tongue weight. In some of the research in the 1970s, they favored putting 25% of the tongue weight on the front wheels (that would mean taking the distance the front when up, and making the front go down by 55% of the distance (all numbers for 144.5 wheelbases and 66" ball to rear axle). That is, if the front went from 37" to 38", adjust until it is 36 7/16".
I have no idea which works best and it is probably tow vehicle dependent. The key, to some extent, is how much understeer the truck has.
The other factor I'm still researching is the WD bar stiffness. The bar is a spring; some early designs actually used a leaf spring (there is a patent application for one on Google). It is much like the truck's springs. A tall soft spring and a shorter stiffer spring may have the same installed height for a specific load, but an additional load will compress the stiffer spring less. When the angle of the truck to trailer changes in the vertical plane, this spring is resisting it. A stiff WD bar, even if set to the same initial tension as a softer bar, will provide more resistance to this movement than a softer bar. There is no advantage for that, and maybe a significant disadvantage. At this point, I don't have a theoretical model, testing data, nor calcs or testing data of actual WD bar stiffness. My guess is having a WD bar specified for a14k trailer on a 5k trailer won't have a good result.
In college, I was told a story about Stephen Timoshenko, the father of engineering mechanics. He had just passed, but several of the professors had been students of his. The story goes that he was called in to determine why a large industrial crankshaft was failing. He determined it was being excited at its natural frequency (e.g. the Tacoma Narrows Bridge). He could make the shaft thinner or thicker to change the natural frequency. He thought, anyone could say making a shaft that was breaking thicker, so he said make smaller.
When trailer instability raises its ugly head, the natural response seems to be more tongue weight...until arriving at a point where the tow vehicle can't handle the load. It may very well be, and IMHO think it probably is, beneficial to get the disconnected tongue weight down (probably 6%-7%, depending on the WD bar adjustment, with a net tongue weight of about 5%) with probably the softest bar that will work. But I can't point to any research to verify that theory. Yet. The goal is to have the tow vehicle axle loads, tire pressures, and relative front/rear roll couple such that the truck is stable with the trailer attached. For a tag-along trailer, a 4-bar linkage hitch will dramatically reduce the forces the truck has to deal with.
