Tow Pro: Can Single Axle Van be Mis-Made?

I bought a single axle pop-top van from an owner who advised that it tended to sway badly when towed over 80 kph, even with sway bars attached. Privately, I thought he had loaded it incorrectly, but found that I too had the same experience. I wrote the van off 5 days into our first long trip - in the NT, near Larrimah - after speeding up to pass another van. On slowing down, my van "snaked" violently, dragging the car around, then it flipped! Luckily the van lifted off the tow ball, both safety shackles broke and the tow vehicle, a Berlina, headed off the side of the road. The car survived a trip down a two metre embankment and a little circuit though the bush, so no one was hurt. Were we lucky or what!! I later bought another van, brand new, almost exactly the same size and layout but a different make (Coromal). Never ANY sign of swaying in over two years of use! The salesman thought that our previous van must have had the axle in the wrong place, because none of his vans had ever had that problem reported. Is it possible that the previous van could have been mis-made?

Bob Moodie, QLD