Hi willi4nd - I'm a bit dense most days now I understand what your rig does. Once the WD hitch is adjusted correctly, and your truck tires inflated to max sidewall psi on the rear and normal psi for the weight on the front when towing - if tracking is not reduced to acceptable level or eliminated I would have the alignment of the trailer checked and ensure it is correct.
It has probly been mentioned in the posts above but when you squat the rear of a vehicle with weight and lift the front, the castor of the front wheels is reduced so the steering has less ability to self center and go straight...
Best to make one change at a time so you know what is causing what result.