Hi Emmerson
We have been touring France for a number of years. Always in late July and August as we are both teachers. Even though we have a long, twin axle van we have never found any difficulty finding a site. We have rarely booked although if we particularly want a certain site we may phone ahead on the day. There are just so many high quality sites in France including the municipal ones found in many towns. Most of them are far superior to those on offer in the UK and at very reasonable prices.
During the second half of August the demand for pitches, also, noticeably decreases as the French hols draw to a close. I too would strongly recommend Caravanning Europe from the CC and Alan Rogers "France" is, also, useful.
Another advantage of not booking is that you be choosy about where you stop and do not have to worry about getting to a certain place by a certain time.
I'm sure you'll have a super time. Happy 'vanning,
Tim