I appreciate the input. This was not my first trip with this rig. I have put 9000km on with this set up, I log every trip and pay a great deal of attention to how it drives each time and had tweaked my set up during a long trip to Washington DC earlier this year. Subsequent trips continued to feel good.
I promise my rig is set up for 10500 and yes I estimate my wet weight at 8500lbs... Still very much within my tow rating. Driver fatigue was not the problem, I did the trip over three days specifically to avoid that issue.
The sway was worse with a full tank of fuel so weight does seem to have some involvement, but again it was only on the third straight day of towing each time. Full tanks on day one and two did not appear to impact the problem.