What shows up is 2 green lights and one red light whether the mains switch on the van is switched on or not. This is not one of the configurations on the front of the tester.
The guy at the campsite says it happens with UK vans and there is nothing to worry about. It is just the different wiring.
Sometimes in Spain and France the live and neutral,are reversed in which case many people make up a polarity reverse lead, there are some threads on this forum.
In the case of the reverse polarity you may find that if you plug an appliance into the 13amp socket it might still work even though the socket is switched off ( Assuming the socket has a switch)
I am surprised that your tester does not tell you what the red light means, do you have access to another tester, perhaps from a friend
I am sure there will be someone on this forum that will be able to help you
If the polarity is reversed then the tester would indicate it.
If the polarity is reversed an appliance won't work if switched off but the cable to it will remain live unless the switch is double pole.
That 13A plug in tester is OK for most faults but it won't display several quite serious faults.
It can't indicate a reversed neutral and protective conductor, or, if the earthing is TNC-S (which it shouldn't be on a campsite) it can't detect if the line conductor and the PEN conductors are reversed.
It can only indicate if an earth is present but not if it is low enough resistance to disconnect the supply quick enough to ensure safety.
It is difficult without being on site to properly diagnose what is causing the problem any suggestions would be pure guesswork!
The best thing is to test your RCD to ensure it works correctly and if you are concerned don't mess about with anything while it is plugged in or switched on and unplug anything that is not in use.
The RCD is your protection against electrocution!
saxo1
Thanks everyone for your thoughts. I have found that when the tester is plugged in the buzzer is intermitant and the red light shows constantly whether the plug switch is on or off. When it is on, the two green light also shine constantly. When the plug switch is off the green lights go out just leaving the red light on. The caravan site (from what I can understand of their English) say it is to do with the way the earth is wired and not to worry.