Our experience is completely different to Emmerson's. We have ALWAYS holidayed during the last two weeks in August, and in around twenty years of caravannng have never not found a pitch in the areas we want to be. However, we choose inland sites, usually part of the ACSI Card scheme (
www.campingcard.co.uk) or Camping Cheques (
www.campingcheque.co.uk) and for overnight stays use municipal sites. We don't try to find pitches on busy beachside sites in the Vendee, the Riviera, or around attractions such as Disneyland Paris, we're happy with our riverside sites in the Ceze, the quieter parts of the Dordogne, the Massif Central, an even (certainly after the end of the third week in August) along the Languedoc coast. We have paid as little as 10 euros per night in the last two weeks in August!
If it's in the ACSI or Camping Cheques book, and offers dates from the middle of August, then you shouldn't have a problem - BUT if you're heading for the only ACSI or Camping Cheque site in a 100 mile radius, then ring up as you travel, to make sure they have pitches available!