Not enough tongue weight is a major cause of fishtailing. It happened to me several years ago. I was trailering a 65 Impala SS convertable. It was an original 396 car. Nice red car with white leather. The engine was out and disassembled; it was in the trunk. I should have loaded the car rear end first. I had about a 30 mile freeway tow to do. About half way home there was a slight down hill. It started fishtailing and I lost it. This was at noon on a Saturday, so the traffic was pretty heavy. I ended up in a complete jacknife with the car heading south in the north bound lanes. I was blocking 2 lanes of the three. Luckily no one hit me and I did not hit anyone either. What a mess. It messed up the Impala, my buddy's car and trailer, that I borrowed.
I am glad that you had better luck on your trip.
