Archie, most people that pull 5ers will say that they are immune to the sway of a bumper pull trailer.

I had a 26' Gulfstream 5er (8K lbs.)pulled by a 91 F450 Super Duty diesel, that got a little sqirrelly one day. We were on the interstate running 65-70 MPH, roads were dry, no wind, no trucks within a mile, when for no reason the trailer swayed back and forth twice and then straightened out on its own as I was reaching for the brake controller. I stopped and checked everything I could think of, tires, brakes, warm hubs, and found nothing. We pulled this trailer with the same truck for 25K miles and never had it do this again. Sort of like a "pay attention" wake up call.
My current rig is a 99 37' 5er pulled with a R-Model Mack, so I don't think I'll have to worry about any instability with this rig.