Although the Captain is right, there is no need to make up another connector, it will save you a load of time is you make one up now and keep it in the locker of your caravan. I made one up about 10 years ago and have used it at least once on every single trip to France during that time.
When you connect up for the first time, you use the test plug (a 13amp plug with 3 LEDs on the back) to tell you if the polarity is reversed. If it is, you use your adapted connector to reverse the reversal, if you catch my drift. If on the next site you find everything is the right way round, you use your normal adaptor, not the reversed one.
There used to be continental sockets only in France and so I always used one of my two adapters. However, many sites are now fitting the European standard sockets and so I guess I will have to make myself another reversed adapter using the normal blue 3 round pin plugs and sockets. It's only a few quid and will save me loads of time over the coming years.
It's not actually the end of the world if your run your van with reversed polarity, just as long as you don't fiddle with switches, plugs and sockets while they are live. Lightbulbs, fridges heaters won't mind too much at all.