I think you are moving in the right direction by adding tongue weight. my 2015 f250 pulled my trailer this past weekend with my rear fresh water tank (50 gal) filled and experienced more sway than I ever have in the past. in fact even with it empty I noticed cars passing me more than i did with my previous TV (Toyota Tundra).
I think the Tundra needed more help from the WDH bars than the SuperDuty needs. I just figured I would empty the tank before heading back home. I also decided to loosen up the chains by having 6 links under tension vs. the 5 links I've been using.
The ride home was the best I've experienced so far! didn't feel anything from passing vehicles at all! I had measured everything and brought the front within 1/8" of empty measurements. But it seems I took too much weight off the tongue.
A trip to the scales is next to confirm, but judging by my return trip, I think I'll find that I was missing tongue weight compared to my Tundra which needed help in that department.