What the OP understands... Is that I have experinenced on two specific occasions driving challenges that were not consistent with approximately 9000km of towing time. Something was different and I am attempting to figure out what might have changed on those two days that effected the vehicles performance. Could my set up require tweaking, of course it could, no hitch setup is going to be 100% perfect. That being said, I have followed the manufactures specs for setup and remeasured during this year to ensure I was still getting appropriate transfer from rear axel to front.
Could I benefit from knowing my weights, of course I could. But I do trust the certified weights I was provided by the dealer and am very cognizant of what I carry in my rig. A search of persons who have posted details of the Sunset 31SS tongue weight I have found results between 750 and 850 (Within 10-15%). I will be taking Turtles advice on how to check mine and expect it will exhibit similar results.
Fatigue was my term, because to the 99% of the populaton who are not engineers, It means tired and in need of rest. I admit to not being an engineer and wondered if parts such as my torsion bars, shocks or suspension could in fact simply be tired and in need of rest after 3 straight days of towing. Based on my driving history with the vehicle this is a fairly logical hypothesis, one which an engineer could probably confirm or deny for me.
And as for being ok when everything is NOT severe... My driving conditions were actually far more severe when it was driving well. When I experienced the most significant sway I was on a Pin straight section of highway in Georgia on a clear sunny day, travelling at approximately 65 MPH, a speed which I have been able to consistently tow at without challenge. If is had happened when things were severe... I would probably had a very different hypothesis.