Just speculation, but I wonder if it has to do with the short wheelbase of your towed vehicle, coupled with the high center-of-gravity, and the short wheelbase of your RV. Maybe this combination makes it more sensitive to things - even like the crown in the road.
I know you don't want to hear this, but I just got my toad on the road... it's a 2002 Pontiac Grand Am that I am pulling with my 28Z (actually it's just a couple of inches shy of 30ft), and there is no tail wag whatsoever. I was a bit concerned after reading your problems, but I didn't do anything more than you have done.
I paid particular attention to the towbar height, and bought a drop receiver that provides just a slight incline.
As well, I did find one thing goofy... the baseplate sticks out on one side about 1/2" more than the other side.
I installed the baseplate myself, and found that even the frame of the vehicle was a bout 1/4" longer on one side than the other. So when I aligned the baseplate, I aligned it with the pivot points of the front wheels rather than the frame or fascia.
So in reality, the front fascia is not exactly symmetrical, and while no one would ever notice it in normal use, a baseplate sticking out the front shows that it is off a bit - in reference to the fascia. But it is aligned to the suspension.
Probably a matter of manufacturing tolerance.
So, one other thing you might do is to measure from the connect points of the baseplate back to a suspension component on the frame (tire pivot point, frame bolt, etc).
You did not specify who installed your baseplate, but if a dealer did it - my experience is most of them are no better than the RV manufacturers... it might be the baseplate was aligned to the fascia rather than the suspension.
The baseplate install guide did not mention this at all, but it was something I discovered... one reason it took 3 days to install the darned thing. I did call a rep at BlueOx and they said it probably can be off a small bit without bothering anything, but that only tells me there is no consensus on the topic.
But if the baseplate is not aligned to the suspension, methinks it might induce tail-wag... but maybe not. You might be chasing a ghost due to my bad advice, however, it never hurts to check-see if it is aligned or not.
One other thing I did though was to use hitch immobilizers to both the motorhome receiver and the drop receiver. There is a lot of play in that area, and it may or may not fix tail wag, but at least is is not rattling like it used to.