Hi,
My first ever post but saw this so can throw in a couple of ideas.
My sister lives in France, so for the last few years we've always driven somewhere and she's met us at our final destination. Like you, we have always had to go in August due to our son being in school.
I've never been to the med coast because it's to far round and doesn't really appeal to us. However the Aquitaine and its body-boarding beaches have done and have been there a few times.
Regarding booking, prior to our son going to school we never booked, just turned up. In school holidays it depends on the type of site you're looking at. Being a family, if you're looking to go to a typical family campsite on the coast with all the activities to keep the young ones happy (e.g. with water slides etc.), then you will really need to book to guarantee a pitch. In the past we've left it late (late being April!) and our preferred sites have no availability so we had to go with our second choices (for our final destination at least). If you have a site in mind that you like then I would book it Jan/Feb at latest. If you're staying more inland then you can probably get away with leaving it later and maybe just turn up. We always book direct on the campsites website and they normally ask for a deposit upon booking and the balance is paid on arrival, though some do ask for full payment in advance (but not many).
For the sites we stay at on the way down, if I do book I generally leave it later as they don't all get full. We have just turned up but for ease we mostly plan the route ahead and book. I was chatting with someone in Contis Plage who was travelling back from Spain this August. They used one of the apps to just turn up at a Camper park (there's loads all over France, literally a car park for campers). They literally got the last spot as it was on the coast.
The main family coastal sites in August are mainly full, though if you leave arriving until late August (i.e. the last week) then you will find availability more easily and the pricing can often be cheaper, despite it still being August. I understand a lot of the French go back to work then.
We've always been welcomed in France, the main rules of the road being ensure you have to have reflective jackets for ALL passengers, a spare set of bulbs, self breathalyser kits, warning triangles, headlamp beam protectors. We always take the beam protectors but rarely put them on as we're always day driving.
I happened to draw up an itinerary for my sister in law this year (she came with us) and this is what we done, The village near Camping Saint Nicolas on the way back with it's monastery was an excellent stopover and I remember there was a camper park there where you could just turn up.
16.08.19: Evening drive to Kent, pub camping (Britstop)
17.08.19: Eurotunnel Folkestone to Calais Depart 08:20 (35 mins)
17.08.19: Drive to Camping Le Sabot, Loire Valley for 2 nights (5 hour drive)
18.08.19: Leisure Day – maybe visit one of the many Chateaus such as Chateau Chinon
19.08.19: Drive to Camping Lous Seurrots (4.5 hours) for 9 nights camping
28.08.19: Drive to Camping lac De Saint Cyr (3hr 50 mins) for one night LINK
29.08.19: Drive to Camping Saint Nicolas (3hrs 15 mins) for one night LINK
30.08.19: Drive to Camping Saint-Louis near Calais (2 hours 45 mins) for one night
30.08.19: Last minute shopping at Hypermarket near Calais (booze run)
31.08.19: Eurotunnel Calais to Folkestone