I understand there are loads (just ordered the Michelin camping France 2011 book) but do you need to book in advance or can you just turn up to find there's normally plenty of room?
Most towns will have a municipal camp site, basic, cheap and clean. My village has one next to a stunning outdoor swimming pool with cafe and restaurant. In the south the standards are generally very high. Very rare to get caught out without a pitch for the night except in the very height of season or if a fete is taking place in the town you turn up at.
Never had to book any French campsites, though I think a 3 family group of us did book one campsite for a week or so.
Don't arrive too late. We arrived at one to find it closed (with a barrier) at 19:30h, so instead we had to stay in an alternative building accommodation, which turned out to be somewhat flea ridden. My wife and son were scratching badly afterwards, but the beasties didn't care for the taste of my blood .