Toilets at most sites are exactly the same as here so no worries.You can by a polarity tester that plugs into your van and tells you if all is ok ( not expensive to buy )reverse polarity is fairly common over the water and means that equipment may still be live after being switched off.Its easy to overcome just buy the male and female plugs like those on your mains lead and a short piece of cable the wire one plug correctly and the other reversed,then you plug your normal lead into the van and the new cable on the end of that and then to the electric post and recheck your tester,sounds complicated but its not just the way I have explained it!