Split charge relay question

[RPWC]

On my van I have one of these, very basic, live and negative input from vehicle battery, and live output from other side, to aux battery, which powers a 12 v water pump.The earth on aux battery is connected to body. I have had trouble with van battery going flat a couple of times over last few days, went to local auto electrician, who tested the battery and charging system, and could find no fault, battery holding charge, as it should considering its only about 7 weeks old, alternator working as it should, and no discharge on the system. He suggested relay at fault, and to disconnect it when not using the van,I did this when I parked the van up at 4.30, just the positive side, when I came to move it on to the drive about 9.30, it wouldn't crank, jump started it from aux battery, started straight away. When I put a meter on the cable from the relay, with engine switched off, it was reading 12.83 v,. Am I right in thinking, that there should be no voltage at all with the engine switched off?Also how can the battery go flat if the live feed from relay was disconnected? can the earth still drain it?