I'm going to have to disagree with your narrative. If in fact the frame of the truck is intended to be rigid (which I agree with) and if in fact the bed of the truck is relatively rigid with respect to mounting to the frame (which I mostly agree with), then positioning of weight is actually quite accurately that "fulcrum" concept. And in fact, as you move rearward behind the axle, you've added more arm length to that fulcrum. Now, I want you to recall that I never said "how much" the AUH should derate the truck. However, even in the apparently misunderstood threads concerning the movement of the GM "ball" (which is NOT the mounting of the assembly to the frame), actual weight measurements proved that moving that ball backwards increased the applied force by a resulting increased weight measurement at the pin - with the same exact trailer.
Now - my beef with the AUH and "derating" is that it doesn't move the application of weight an inch or so. It seems to significantly distribute that weight further back along the surface of the bed - at least back to the very back of the rear wheel houses. Now, how much weight does it move back there? We really don't know. And Anderson doesn't even mention it. Let's face it. Look at over the road tractors. Exactly how many of them have that pin all the way back? That would be none.
Now, when you combine this unknown change to weight distribution with people even using them in Cummins powered 2500's that already have a narrow payload window, to me it's like rolling the dice. Pulling a 9000lb trailer with a truck designed to PULL 18000lbs does NOT give you leeway if it's a gooseneck or 5th wheel trailer, on a truck with already limited payload. Then, you can even make it worse. If you then add air bags to stiffen the rear end, you're now lightening the front end of the truck further.
I do agree that the major issue with the design is - well - the design. We can argue all day long about how many AUH failures are the result of improper installation, etc. But we don't see those kinds of failures with B&W installed hitches do we? Gooseboxes? To me, if it were structurally sound (which frankly I don't believe) then one could just try and estimate the impact of that distributed weight. I believe you still should have to. I honestly think that there should be a very VERY caution and warning telling people about the likely change in applied pin load. But for me, it's like I said. Plenty of people have used them. Plenty of people drove Ford Pintos. A blind man can walk across a busy street and not get hit. That doesn't make any of those decisions "smart" - it just makes people a little luckier. Maybe your risk with the AUH isn't all that great. But I completely and totally believe it's a higher risk than with a traditional hitch assembly - and towing heavy is simply not something that I'm willing to take unnecessary risks with.
As I said, the physics is more complex. Yes, you add more length to the fulcrum as you move rearward past the axle, but the bulk of that weight change is around the fulcrum of the front axle, not the rear axle. The frame and engine are levering down on the front of the truck. Wheelbase of the truck is 149 inches. For example, let's say you have 10,000 lb trailer with 2,000 lbs on the kingpin, 2" in front of the rear axle. Torque around the fulcrum is 2000 lbs x .167 feet = 333 ft lbs. That puts 333 ft lbs of forward torque on the rear frame AT the axle - - down force ahead of the axle. Move the kingpin (or weight center) back to 2" behind the axle, back 4 inches, now it's moved to 333 ft lbs of rearward torque.
Now consider that the frame is rigid, as you agreed, it does not bend. The front curb weight of the truck keeps the truck from tipping backward due to forward torque of all the truck forward of the rear axle. If I recall correctly, the 2500 has about 65% of its unloaded weight on the front axle. I'll simplify and call it somewhere over 4,000 lbs of front axle weight. That's centered 149 inches ahead of the rear axle. The downward torque from this load rotating around the rear axle is 4000 lbs x 12.4 ft = 49,600 ft lbs of forward torque. Moving the trailer weight backwards 4 inches this is going to have little or no rotation effect on the rear of the truck. Sure, it counters a
minute fraction of the front end weight, about 0.7% if you want to estimate. About 27 lbs of upward force at the front axle to counter the 4,000 lbs of downward force.
And if the hitch is rigid to the frame, you can't claim that the weight is redistributed to the back of the hitch body. It's distributed along the front of the hitch frame too, and any physicist or engineer will tell you that you'd calculate it at center of mass at the ball. If the AUH shifts the ball a few inches, then you calculate where the ball is shifted to,
not to the back of the assembly. Because the bed is rigidly attached to the frame, and the hitch is rigidly attached to both, making all 3 into a single rigid structure. (With a weak point in the hitch construction, apparently.) But if the ball moved 4 inches, then my calculations above are reasonably accurate.
Of course, the effect of putting 2,000 lbs of extra weight on the rear axle does add load to the frame and axle at that point, and the rear suspension is going to compress slightly, depending on how it's configured. Putting it 2 inches ahead of the rear axle adds about 27 lbs to the front axle curb weight. Putting it 2" behind the rear axle reduces front axle curb weight by 27 lbs. Putting it directly above the axle does nothing to the front.
Last point - again - I said I didn't care about the AUH one way or another, I am just talking about the physics of the ball mounting location.