The camping trailer had a battery to run the internal 12 volt electrics. That battery is charged from the car charging system via a split charge relay. Basically it stops the camper electrics from flattening the car battery if the two are hooked together. I sent her a European style split charge caravan relay for the 120 together with the plugs.

I don't think the trailer would ever have had it's own registration document, to tow a trailer you simply stuck the car's number plate on the trailer. They are now registered independently in some European countries, in the UK however we still simply use a third number plate.