"Sway" is not "OMG its fishtailing!" Sway is more like a meandering and is most commonly caused by wind, either a cross wind or a passing vehicle. If you're loaded correctly it should correct itself. If it begins to move for no apparent reason, and does it a few times in the first 10-20 miles at speed, your load is wrong. Pull over and correct the imbalance in the trailer.I don't tow a trailer and know nothing about it so please forgive my ignorance. But looking at this towing guide I noticed it mentions that, while driving, the TBC slider should not be used to independently activate the trailer brakes if the trailer is swaying. I'm just curious, if you are in a situation where the trailer is swaying, wouldn't it be helpful to let the expy coast and apply some trailer braking to help stop the sway and straighten it out? Since they advise against this I must be wrong but can't figure out why.
If its fishtailing.....apply the brakes slowly until it gets back in line, then pull over and check your balance and the hitch. Have a smoke. It'll calm the nerves a bit. If you simply can't get control of it, squeeze the crap out of that dash knob. Don't worry about "upsetting the tow". You're already there.