I would start at scaled weights. [urlhttp://catscale.com/cat-scale-locator]CAT offers a locator, but I'm not sure if they operate in Ontario. Otherwise, most landscape supply facilities have scales that might be worth calling. It will prove you are right about towing weight, but more importantly is the TW% and WDH loading which are the largest affects TT owners can change to affect handling. Most vehicles tow well with overloaded and as you said regardless the combo has proven better in the past.
Since you have logged many kilometers with the setup, there are other possibilities. The truck or TT alignment may be off. Potholes do wonders!