[SCBillandJane]

You have covered many of the obvious possible problems. The only time that I have had trailer sway was too little weight on the tongue of the trailer when I was hauling a boat or utility trailer. Since a Trailmanor has a lot of weight on the tongue by the nature of its design, the only thing left that I can suggest is that your placement of camping gear, water, extra battery(?), generator, rear hitch, or other heavy camping gear has upset the needed balance of weight. The single axle acts like a lever so that weight placed further from the axle has a greater effect than that same weight only a foot from the axle. There is a small possibilty that you could have only 200 to 300 pounds on the tongue which would be very difficult to lift, but not enough to give stability.
Suggestion 1. There is a way to measure tongue weight using a bathroom scale that requires a few other cheap pieces so the scale shares the weight. Maybe someone else knows where to find this method. I can't remember where I saw it.
Suggestion 2. Remove the generator and carrier from the rear of the trailer and see if that makes a difference.