There should be no need for WD since the hitch weight won't make the truck squat much or take much off the front axle.
I have not found single axle trailers to be more prone to sway than tandem axle trailers. A 20' TT probably would not need sway control, provided the hitch weight is in the proper range of 10%-15% of total TT weight. However, there is nothing wrong with having sway control, and it would provide an added layer of safety. An Andersen would work well for that rig. Added benefit would be the smoother ride, less porpoise bounce.
The longer the TT, the more the tail can wag the dog. However, even a short trailer can sway if hitch weight is too light. Long ago when I was young and naive, I towed a 4'x8' utility trailer (about 11' overall length) with negative hitch weight (a steel part hung out the back end)... and after 3 uneventful hours of towing, it suddenly swayed violently when I slowed down to make a turn.
