Nick,
Usually the earth cable is bonded to the caravan chassis, and also each electric outlet has an earth connection in it. Dont forget that the mains hook up lead also earths at both connection ends.
When you say you have a fault, is your RCD tripping out or is one of the MCB's tripping (current overload protection) ?
If the RCD is tripping check that your mains hook up lead is not damaged and also that any of the sockets etc do not have a loose wire.
Another cause could be the element in the water heater which might be faulty ( ie knackered ) and thus going to ground and tripping the RCD.