When you get passed by a big truck, that is a PUSH feeling. It is not sway. Sway is an oscillation.
I've only come close once, with a combination of going a little too fast, an odd road surface, and full tanks, which probably unloaded the tongue a little bit. First thing I noticed was the steering feeling wonky, and at the same time, could see the trailer appearing and disappearing in the mirror on the driver's side. I immediately let up on the gas and it stopped, even before I had a chance to use the brake controller.
