The tow vehicle,
You have to get the towbar in as close to the rear axle as possible, if your gooseneck has adjustment holes, pull the pin and place the neck in as far as possible. Stiffen up the suspension on the tow vehicle, that is all the way from the tyres through to the suspension springs, use LT tyres with stiffer sidewalls, get the pressure up at the max allowed by the tyre manufacturer, can't do much about air spring stiffness.
The van (pig trailer configuration i.e., an axle group under roughly the center of the van),
Make sure that you have nose weight on the tow coupling onto the ball, rule of thumb is 10% of the ATM. There is a study floating around and I think once it gets below 5% you are asking for trouble. Make sure your suspension rubbers are in good condition. Load all your heavy components in the van over the axles, too much weight away from the axles causes a pendulum effect, that goes for weight too far forward or behind the axle. I'd also pump up the tyres on the van. Make sure the van is riding level with weight on both axles if it is a tandem.
Once you have a stable configuration you can start backing off the tyre pressure on the tug for a little improved comfort. Check as you lower the pressures that you still have a stable setup before lowering the pressure any further.
A number of threads on the forum on towing stability, the above captures most of what gets covered in these threads.
There are limits and I'd say that the 3500 kg rating on LR products is close to that limit. More likely to stay safe and stable by slowing down. You don't want to be on the limit only to find yourself coming down a hill and start to exceed it and getting the tail wagging the dog sort of speak.
