We have towed a lot of trailers behind different trucks and campers over the years and like others have said when things go wrong you can feel it or see the side of the trailer when you shouldn't.
Our mirrors stick out far enough that we can see a little bit of the fenders on the horse trailer but when the jeep is flat towed we can only tell it is behind us when we turn a sharp enough corner and catch a glimpse of it.Other than that it is an act of faith that we keep on going down the road thinking we have a jeep behind us.
We have seriously considered some kind of camera to be able see the Jeep and maybe also mount it in the horse trailer to observe the horses.