My father made up a magnetically mounted (on a short post) wide angle convex lorry mirror (set at 45 degrees approx) which he attached to the caravan A frame when connecting up. It was mounted high enough to see, through the rear view mirror of the car. It made reversing to the towball an absolute doddle. I suggested that he patented it but he never got around to it.
A more permanent fixing might allow you to see the proximity of the rear of the car too?