Sat Navs & Co-ordinates

Can anyone please help after much thought and planning and plenty of reading on here we have booked for France next year!! scared now!! We have just been trying the co-odinates out of the caravan europe handbook and have hit our first problem we are trying it on different sat navs but sometimes it seems right and other times it goes somewhere completely different I would greatly appreciate any advice on what others use and how easy or hard it is to get to the sites once over there