Just to add to the chorus -- our plug came out of the socket once -- it did not drag but the batteries did not charge and our turn indicators did not work. Since then, I have always wrapped a small bungee cord around the flap that covers the socket on my truck, so that it locks the plug in place. And I have used long cable ties to take up some of the slack on the cord, while leaving enough slack for the trailer to turn without pulling on the plug.
As part of our last-minute checklist, we check the bungee on the plug and we make sure that the turn signals are working on the trailer.