Hello taffy,
Reversed polarity can be a problem, and if you have means to check for it and correct simply, all the better. In general a short length of hook up cable with a male CEE form and Female CEE form with the phase and neutral wired crossed over is usually all that is required.
Strictly speaking such adaptors are not approved, because the wires are crossed. but it is the only way to correct for an bollard that has been wired up incorrectly.
Most mains appliances will work quite happily on a reversed supply, it only becomes a problem if an appliance develops a fault. If for example, a short circuit occurs, it should blow the fuse but if the supply is reversed then the fuse will only have disconnected the neutral supply leaving the appliance live.
If the short circuit is to earth, then the whole appliance will still be live and in danger of damaging its cables by over current ant its associated heating effect.
Fortunately in all modern caravans (and all new systems in houses) the supply has two other protective devices.
The current will be limited by an MCB (miniature Circuit Breaker)
which is like a fuse but much better and consistent in operation, and in the event of a fault to earth, the RCD (Residual Current detector) will trip. The RCD's fitted to caravans are double pole. meaning that the disconnect both phase and neutral simultaneously. This truly isolates the system and protects you.
RCD's will work with reversed polarity.
The most important thing you can do when you get to site, is to check the RCD in your caravan will trip out when you press the test button. If it does not trip out do not use the supply. If it does not trip it usually means the earth system is faulty.