Dan,
Indeed, from reading many posts on the stability issue, it seems that
a precise, stiff suspension is what makes the rig stable. I recall one
poster saying his rig was not very stable till he fixed a somewhat
worn and loose joint in the front suspension that was barely
noticeable when driving normally, but it improved the stability of the
rig enormously.

I suspect that when the tow vehicle itself is heavily loaded, making
it less stable, it can have the same effect and cause the rig to be
less stable than when the tow vehicle is more lightly loaded.

The Honda Accord I used to tow had a Iow cg and a fairly stiff
suspension compared to many large tow vehicles. The Astir CS and heavy
trailer were clearly stable to 80MPH, which is as fast as I ever
went. Never tried faster as I never trusted the small tires on the
trailer too much.

I doubt stability is about the weight of the tow vehicle.