Some parks will be full, there are many people in other European countries who book months in advance, to a year in advance, to be sure to be on the one park they prefer. Those parks will likely reserve all places they can for those people who book ahead.
Other parks will have spaces they keep open for people comming in on the day or who make a booking a few days in advance, mostly those are places for just one or just a few nights, unless the park is not popular enough to get fully booked.
Your chances on a place in a good park are bigger in years with poor weather in the area, as many people will either not use their booking or not make a shortly before traveling booking.
It will also depend on which part of France, which set of weeks and the kind of park you prefer. What I hear from people at work is the kind of parks with many options for entertainments while still having modest prices. Of course those that have a less good price to facilities balance will have spaces much longer.
I know this is not really a useful answer, as there are so many options.
Your best bet is to call a few of the parks you would be happy to stay and see if they still have spaces for the time you want to be there.