We’ve camped in France for the past 5 years, always in August.
first year we went to La Rochelle and the weather was crap. IMO if you’re gonna drive all that way, you might as well do the last few hours and get to the Med for guaranteed (pretty much) good weather.
Last 3 years we’ve been to Gorge Du Verdon area – stunning scenery, striking distance of Monaco, Nice, Cannes etc for day trips. Some good riding too if you take the bike.
Personally we go for cheap roadside motels if we want to get there quickly, or decent B&B/gites if we want to enjoy a more leisurely journey. Couldn’t be arsed getting the tent set up just for a stopover (but our tent is mahoosive and takes us ages to set up).
Most sites will be getting quieter by end of August as all the French holidays have finished – usually still quite a few Brits and Dutch around though.
When working out travelling times in France – you cqn pretty much assume setting the cruise control at 80 and only slowing down for fuel and toll booths if you stick to the autoroutes. Sooooo much better than UK roads.