I was just thinking. The Tesla cars have cameras all around for the autopilot. Particularly a camera at the rear. I would imagine that any trailer used with the semi will either have to have a compatible reversing camera already on the trailer that could be calibrated, or you'll have to add some sort of temporary 'camera bar' like you do a lighting bar on a car trailer.
Thus it would behoove Tesla to create their own semi-trailers with rear (and possibly side) cameras and other sensors built in.
If they're going to do that then why not cover the roof of the trailer with solar panels...
... and most certainly it would allow for much greater range (or speed, or load capacity) if the trailer also had a battery pack and drive-train.
This, along with some of their snazzy autopilot software could mean on arrival the trailers could disengage and go park themselves (or hitch themselves up) even. At a push (since less aerodynamics) the trailers could perhaps even self-drive short distances such as around or between depots, or to multiple local customers if they were shorter double trailers.
Oh, the possibilities!