Thanks for your responses. To answer some of your questions:
The 12V system was working when I left on holiday but not after that. We checked the batteries, fuses and all were okay so we went back further.
Where the power from the 12V batteries comes through the front wall of the trailer and is attached to a "block" by 2 screws with 2 wires under each screw on the block.
My electrician says this block is to prevent any overload/excess power coming into the trailer. The block itself is attached to the front wall of the trailer and is a piece of black plastic, rectangular shape, about 1" x 3/4" by 3/4" thick.
Everything was checked with a multi-meter and that's where he found the short.