I suspect the earliest used a variety of connections, probably going right back to the old 15A round pin domestic plug/socket and then on to the more modern 13A square pin domestic plug/socket before adopting the present 16A ISO connector. I know the latter was the "normal" standard in 1983 when I bought my first caravan but few had it fitted as standard - I fitted 230V electrics myself to our '83 Ace Globetrotter and got a local electrician to test and certify it - ironically the RCD I bought from a caravan dealer failed his test, not fast enough to trip, so I had to replace it with a unit from my local electrical wholesaler.