This is fantastic – You can cycle the whole way from Bordeaux near the Atlantic to the Mediterranean Sea along green cycle routes (voies vertes) and canals. There is lots of information and maps available at http://www.canal-et-voie-verte.com/index.php. You could do all or part of it.
Visit Carcassone, Toulouse, Beziers and other fantastic French places en route along the Canal du Midi, Canal de la Garonne and Voie Verte cycle route.
Eurocamp and Keycamp offer campsites on the coast 80km from the beginning of the route at Biscarrosse and there are lots of great campsites on and near the Mediterranean to chose from at the end in Languedoc Roussillon.