Going to France in the summer holidays and wanting to go to different places on a whim - you certainly can do it, I hjave never booked a camp site in advance when going camping in france, Belguim, Spain, Italy.
Though the end of July and begining of August will be busier and you may not get such great pittches and possibly only for a few nights, then have to move on. Which is fine if you are touring.
It is also more expensisive at the end of July and the begining of August, later in August (around the 21st) the price will drop. The campsites will be less busy also and the summer is still warm - usually.
Have pais between £20 and £40 per night for two adults and two children on a campsite and the price doesn't represent the quality of the site. The one site that we stayed for £40 was ok for pool but 3 toilets between around 150 people was dire - so we left the next day.
french municipal site can be wonderful, the one at samur is apparently. But the one at Bayeux? is really rather poor and unfriendly. There is a great campsite just out of the town though - about 8k, which is lovely with a swimming pool and restaurant on site and lots of ground to play.
municpal site will not often have pools, which my dd loves so we dont usually stop at them.
Where were you thinking of driving to in france?