Over here in the sticks,
You are not allowed to have a cord or anything protrude from a caravan/trailer.
It must be connected to the trailer with an "appliance inlet" that has male pins and is permanently installed on the side of the "vehicle".
You connect your live source with a female (weather-proof) cord connector, RCD/GFCI protection comes from the source of supply.
For those that have heard of it, the connectors must be rated to IP 56 (dust/moisture proofing).
One thing I would stipulate though, with regard to the flexible cords, is get the heaviest insulation possible, for the size of wires, these cords will get knocked around, run over and generally abused.
I'm involved with Civil Defence over here and our gear gets a really hard time.
