Air shocks

[leegreen]

I have had 3 different air shock setups. They have all leaked down slowly, despite taking much care with installing the lines. They can support weight.
I would worry about having them be the sole suspension, if there was a failure you'd be waiting for a tow truck.

In one instance the air shocks still held air as good as ever, but failed as shock absorbers after only a few thousand kilometers and were replaced with non air shocks to make the truck stable on highway again.

A pair would probably support a few hundred pounds easily. You could make beefy enough shock mounts.
But I would not do what you are contemplating, A couple leaf springs and straight axle makes a dandy trailer with no fuss. Remove some of the smaller leaves from the springs to soften the ride if you are building a light trailer.