Should I make adjustable shocks softer or harder when towing my van?

Google has plenty of information :)

Gary
It depends on a few factors. The idea is to match load and suddenness of vehicle/van dynamic movement. Depending on the vehicle you have and it's load and towball weight. The adjustment isn't really hard or soft, just called that. It is resistance to sudden movement, done slowly not much is felt, worked fast it presents as hard, or in other words more sudden resistance. What vehicle do you have? because hard sudden movement may cause shock mounting damage or splitting of rubber bushes. It is the ability of the shocks to absorb energy and not their ability to simply create "hardness" which is important. If you feel sudden violent harshness with suspension action it indicates too much sudden resistance. The idea is to have it all move and absorbed not simply blocked by instant hydraulic force.

Good shock adsorbers will have negligible resistance to compression, even though they fill really hard when you try and push them in.
The valving is optimised for resisting them extending again.

The idea being that you hit a bump and the wheel is pushed up and the spring compresses, the shock adsorber resists the spring extending again just as quickly and making the vehicle bounce down the road.

What setting? I'd probably go with a firmer setting as you will have a fair bit of load moving around down the back, but it all depends on your springs and the load.

You can sometimes over do it and set some damper too stiff though. The spring doesn't get a chance to extend to its neutral position again before the next bump and the suspension can get rather harsh as the springs are kept partially compressed for each hit. Its generally only an issue on corrugations though.

It’s very subjective. If you have a prado with standard springs and a big van you’ll be cranking them up.
If you have an empty twin cab Ute with a gvm upgrade and towing a light 16 footer you’d be backing them off.
I only had one set of adjustable tough dogs and they were on a competition Ute. We ran them hard inroad mainly to make up for the lack of sway bars and to limit bump steer. Off road, turning them back gave a plush ride.

Depends on what sort of ride your after and how the vehicle reacts on road when supporting extra weight when cornering lane changing braking etc.

I had adjustable shocks on my 4wd but had heavy duty springs to suit so it could support heavy loads when needed. I ran the shocks on a comfy level most of the time but not soft enough so the vehicle wanted to follow ruts, dips etc in our great less than average QLD roads. When carrying a load you can feel the vehicle react quite differently generally making it less responsive and wallow a bit it just doesn't feel right so hook the van up, take it for a reasonable drive before you go away to help select what might suit then you can tweak it later on when traveling if needed is my advice. It's amazing the difference it makes and the vehicle will feel much better when you get it right that's if you have the correct springs to suit as well.