Touring down the west coast of France

as long as you don't crush crops / disturb the cattle / light fires on the ground ... find an isolated place and pitch up
france is such a sparsely populated place you should be able to find a nice quiet spot to rest your head where no one can spot you

Keep low especially in touristy areas,
dune du Pyla/ Cap Ferret is a very obvious one,
I would camp away from it.
Remember that unless you make yourself conspicuous,
local cops or gendarmes will make their rounds in cars..