When did electric hook-ups start

Hi all, Just been chatting with my wife about caravans in the 1970's when we first started. We had a Sprite Alpine 4 berth with a bunk bed made up of 2 metal bars that dropped into 4 cups and a material cover for the bed covering. we remember gas mantles and foot pump for water, but no electric hook up, we were wondering when we first started to use electric hook up, has anyone any idea of when they were first introduced, just to settle an argument, thanks