we have a caravan and belong to a number of related forums. a lot of campers - tents and caravans - don't even bother booking in france for the peak summer holiday months and reckon there are always pitches available. while i'm sure this is true as so many do it this way, it's not something we would take a chance on, particularly if we know where we want to go and which sites we want to stay at, so we always book in advance whenever we head across the channel. we go for may and june, and september and october.
on the other hand, easter is only really the beginning of the season and not nearly as busy as the school summer holidays so i would imagine, going by what people have said, that there will be plenty of spaces. also, france is (obviously) a lot bigger than the uk and a lot more geared up to the whole camping thing, and has loads more sites.