The video shows my X22 at about 75 MPH with torsion suspension. Straight as an arrow and no leaf springs. I have a similar video of my HQ19. Perfectly stable with swing arms.
You're right that stiff leaf springs do well on smooth highways, but they suck off road because they are generally stiff, have a lot of stiction, have no (or worthless) shocks and have limited ground clearance. So, they bounce around on rough roads and beat the trailer up with it jumping around or vibrating from washboard.
Since this thread is about off-road caravan style trailers, it makes sense to talk about what works on rough trails instead of on smooth highways.
Swing arms can have almost no stiction, much more travel and have quality shocks. Black Series puts shocks on that are worthless, but that does not mean swing arms are worthless, and it is easy to get good shocks as a direct replacement. I did it on my HQ19. Sway bars are an excellent addition to suspensions that actually have good travel and good damping, instead of just being stiff. That why all cars have them and why they are on the front of pickups. The front is soft and tuned, the rear is generally just stiff. Even the car manufactuterss know the difference here and get it right.
Off road, it is important to keep the poundingt that the trailer receives to a minimum, and keep the tires in contact with the ground as much as possible. And I'm not talking about speeding, or racing. I'm talking about the quality of the ride, to minimize damage, and bouncing around. One place go in Death Valley is over 50 miles in on dirt and rocky roads, each way. If I can only manage 10 MPH, it takes over ten hours to get there! Each way! If I can go 20 MPH average, I can cut that in half. Arguing to just slow down, when the speed is already at 10 MPH, is ridiculous.
When a trailer works well off-road, it might need some help on-road to keep it from leaning too much on turns. This is where the sway bars come in, although they doo help on dirt too. They help keep it stable both on and off road, but probably more on-road. Can you imagine buying a new car and installing leaf spring suspension, and then trying to argue it was more stable on the highway, or better on dirt roads? No way is that true and that is why cars and trucks have coils springs, at least in the front. Much better ride, much better cornering with sway bars that tune the suspension, much less bouncing around. It works with cars and modern pickups the same as it does with trailers. Try riding in an old Dodge Ram from the eighties and convince yourself it is better with leaf springs.
I've graduated from archaic leaf springs, am willing to pay the price for quality and I spend a lot of time on rough roads. To each, his own. But arguing leaves are better because they work better on the highway, when we are talking about off-road trailers, simply misses the point. And suggesting that leaves are better because companies install junk shocks, is more of a comment on the manufacturer than the basic design. Black Series is generally an example of how not to build. And we didn't even get into the variable ride height and leveling available with air bag or hydraulic systems.
