If this is the same whichever tap you use try this
Turn on 12V master switch and turn the taps on
Using a meter start at the socket on the side of the van which should show 0V between the two wires (you said it works on your friends) and then follow the cable back to the next connection and then the next until you get 12V, this will indicate a broken cable or more likely a bad connection.
Without sight of the wiring diagram that is all I can suggest at the moment as there may be relays involved and I do not know how your switches operate.
At a guess the most likely point might be the connection on the back of the socket or a dirty connection in the socket as these are most likly to suffer from corrosion - a squirt of WD40 usually cures this for a while but make sure it does not go near the water inlet.
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