Make sure that EHU is disconnected from the 'van, then simply remove the connector from one of the battery posts. Connect a multimeter set to Amps between the disconnected cable and the battery post. Start on a high Amps setting (in case you have left anything switched on inadvertently, and work down to the lowest range. There may be a very slight leakage but I would start to be concerned if there was an unexplained current of over, say, 3mA.
It is marginally safer to do the measurement on the battery negative side ..... just in case you accidentally touch the caravan metalwork with a meter probe or spanner.