You do not need to reserve in advance unless you really want a specific camping.
Not everyone reserves a spot, as that often costs money.
Not reserving is a risk, but a small one.
I often go to France without a reservation,
like I did two weeks back.
It's relatively easy to find a spot, especially if you are willing to camping-hop.
That is not required at all though, I've successfully found spots for two weeks often enough, even in popular destinations, but it may take two or three tries in peak seasons.
If a camping is full, it will often have a sign outside saying "complet" (full).
I own a Michelin Camping Guide that will tell you the locations and price ranges of many campings in an area, which is very handy when traveling abroad without the Internet and no reservations.
They cost 10-20 euro.
travel.stackexchange.com/users/36405/Belle
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.
travel.stackexchange.com/users/27650/Willeke
A third option which works if you're not very fussy is to book very close to the time you want to stay.
Web bookings are often an option but not at the real last minute.
Phoning just before setting off
(i.e. "have you got a space for tonight?") can be very effective in saving you a wasted journey.
At least at the bigger commercial sites the reception staff tend to speak good English if you start trying to get by in French.
This is less likely to be the case at the municipal sites, some of which can be very good.
As we're rather demanding about facilities and location, we generally book ahead; we also travel outside the French and English school holidays.
In that case most sites we've stayed on have some space available, but I wouldn't bank on arriving without a booking in the school holidays unless the weather was very bad.
travel.stackexchange.com/users/18491/Chris H
I don't have an extensive experience
but so far, I've seen it depend on two factors:
if you camp with a tent then that should be fine, in particular if you have no car.
The campsites that become unavailable first are those with mobile homes and those for RVs/vans.
If you don't particularly care about a specific spot or have no specific vehicle then it will more likely to be fine.
if you go to city campings then it is more likely to have spaces.
City campings ("camping municipal") are usually less classy, more a public service.
This is not as fancy, clean and might not have a swimming pool like 3 or 4 stars campings.
But at least it might have vacancies.
travel.stackexchange.com/users/3044/Vince
You don't really need to book early but it may be worth considering depending upon where you are planning on going.
Camping is an extremely popular pastime in France with many different nationalities going there each year however there are also literally thousands of campsite located in the country.
There are plenty of places that you can find great campsites at including CampingFrance.com and GoCampFrance
travel.stackexchange.com/users/53036/tim
I have been camping in France a number of times and haven't ever booked anything in advance other than the first night, when I have arrived in France early evening and wanted to make sure I knew where I was staying (even that would have had plenty of space).
However, my experience is of going in May and June, and in May and even early June many sites were not yet open.
Having said all that there are so many campsites in France that it's hard to see that they would all be booked up at the same time, even in high season.
For me it's part of the adventure — "time to move one...
I think I'll drive south for a while..."
travel.stackexchange.com/users/81946/Jerry
I might be camping in France a couple of days this summer, but I don't have any plans regarding the trip and this will happen if am close enough to the park.  Is it ok to just go to the park or is it advisable to book in advance during summer? Can parks be full?
Do I need to book in advance in French camping parks?
travel.stackexchange.com
<france><bookings><tips-and-tricks><camping>
travel.stackexchange.com/questions/76132
travel.stackexchange.com/users/3736/nsn
Some parks will be full especially if you have a camper van/RV or are coming to a popular park during peak season, but especially if you have a tent and no car or you are coming in the down season, you should be able to find a spot. Try contacting a few parks in the area to check availability, or book closer to the time. Some people book long in advance but this is quite rare. These are probable camper van and mobile home bookings, as those pitches tend to book up first. Small tent pitches without a vehicle are usually easy to find. It may be a good idea to make a booking for the first night, just before you arrive, to be sure you have somewhere to stay. Lesspopular campsites tend to include those in big cities. 
Some campers book a long time in advance but campsites often reserve some spots for late bookings. Booking is not considered necessary unless you want a specific campsite. It may be a good idea to book just before arriving, especially if you want to go to a popular site. Perhaps it is a good idea to book the first night, to be sure you have somewhere to stay.  If you are camping in a tent and travelling without a vehicle, you will probably have no problems. Camper van and mobile home pitches tend to book up first. Also, city campsites do not tend to be as popular.