Many of the coastal sites do close down at the end of September - and seaside towns tend to 'shut', meaning that many facilities are not available. You'd probably be better at an inland site. I can recommend La Citadelle, at Loches, in the Loire, or Le Croix du Vieux Pont at Berny Riviere, near Soissons. Both of these are open in early October, and both taking Camping Cheques, so