There is no standard for construction of a semi-trailer, it differs quite a bit depending on expected use. A significant proportion of the boxes now on the road are overseas shipping containers, which are very strong, but heavy. At the other end of the scale, lightweight frames with fabric walls, not meant to contain the load at all, simply enclosures to keep out the air flow.
In between, metal wall trailers, it depends on the manufacturer and intended use, whether or not the trailer is refrigerated, whether or not it is meant to contain the load or just hide the load. A $10,000 trailer is not the same as a $50,000 trailer. Some trailer walls are load bearing, some are useful as tiedowns, others simply enclose a space for a load supported from the floor only.
If you find a trailer built to be a moving van, you could probably hang just about anything you want from the walls.