What is the net weight of a 53 ft steel trailer?

Much too heavy for practical use. You'd have to specify what type of trailer you had in mind. As far as all steel construction goes, the only ones you'd find are 53' flatbeds and stepdecks, armoured van trailers (very uncommon, but they are out there), and equipment trailers (RGN lowboys, RGN pontoon trailers, RGN beam trailers, etc.). Weights vary greatly between them.

As for a typical 53' van, the frames are steel, but the floor will be wood (or some use polymer), and the walls will typically be thin aluminum.