Something like this might work:
https://tailgatecamera.com/collectio...irror-3-inputs
It is a replacement rear view mirror that is a dual LCD screen. If I were to replace my side mirrors, this is the type of screen I would use. It doesn't use any extra instrument panel real estate, and I suppose it could be wired so it is on all the time while driving (on with the key on). The one I linked takes 3 inputs. One input could be from the rear of the trailer, the other two would be from the side. The problem is where to place the two side cameras. They would need to be far enough out so you could see the same point 200' behind the trailer as required by several state laws. I don't have a good answer for this.