Camber angle on a two-wheel trailer? I am not aware of any two-wheeled trailer with (intentional) camber. Are there any? If not, why don't two-wheeled trailers use a little bit of negative camber?

I think it would reduce the stress on the wheels in turns and also slightly improve stability (due to wider base). A small enough amount would not affect total width nor usable area as the wheels are widest at the hub.

Race cars used to use camber, sport wheelchairs use it (I think regular ones don't only because it would make them wider), so why not trailers?

I am willing to believe it causes an increase in rolling resistance, but I don't think it is significant enough.

I've been thinking about this for a few weeks now and I still can't see a reason why it would not work.
Tyres wear a heap faster when they're not square to the road surface.  For example, tadpole recumbents wear out front wheels very fast.

Also, those riding bikes while towing trailers are going relatively slow.  I've managed to roll a kiddies trailer that was loaded with two landrover road wheels weighing 68 kilos, but that took a speed of around 22 km/h on a fairly tight roundabout.

Certainly it could work - you'd want wheelchair stub axles and mount the receiver tubes at a slight angle.

Downsides, your wheels would suffer from sideways stress toward the middle when travelling straight and standing still.

Also, the wheel bearings are going to be under more stress all the time too.

Racecars and race wheelchairs are about performance, not longevity, so a different set of requirements.