Please excuse the ugly napkin sketch, it was drawn on my potato and not scale.
Back of trailer and channels the door frame ride in not drawn.
Winch hard mounted off the door frame. cables tight during open/close, and have stops that don't fit through the passthrough tubes in frame when it hits horizontal so its never going to slack.
Lift actuator not drawn but uses lugs at bottom of door frame, and another lug on a support whereever it fits coming off trailer frame.
Then possibly another winch down gate for the pretty side that shows when it's closed, or a big set of doors like a normal cargo trailer to hide this mechanism when all closed up.