So if your weights in the trailer was not changing much like carrying water in the waste tanks to cause a change in sway then maybe your sway control was changing somewhat day to day. Possibly the weather was changing too. I think some of it might just be perceived changes like just after a rest stop. Maybe you are more sensitive to the moving after stopping for a little while but after a while of driving you notice it less. I know you are setting your tension bar the same each day but is it possible that road grease build up on the bars reducing it fiction enough so that sway control got worse.