There was a very tragic story posted on here a while back where a family was towing a gooseneck horse trailer and they left the road to avoid an accident (I think). Unfortunately, the trailer came through the back of the truck and killed all but one person, or perhaps all of them.
That being said, I've never read a story or seen where a fifth wheel RV disconnected from the hitch and came through the back of the truck. In my mind, I would find it hard to believe that this could actually happen short of someone driving full speed into the back of immovable object. If this happened, the 5er coming through the back is probably the least of your concerns. Under any other imaginable circumstance, I believe the nose of the 5er would more/less self-implode upon impact and the frame would get caught up somewhere along the way.
The horse trailer mentioned above was probably built more rigid than the truck itself and likely weighed 20K-25K pounds.
