Sites on the way down:
1. Chateau du Gandspette at Eperleques just near to Calais.
2. Lac de Liez in Champagne/Ardennes region
3. Lac de Panthier at Vandenesse in Burgundy
4. Camp du Verdon at Castellane in the Verdon Gorge
and many others!
All the above take Camping Cheques, and will probably be open in September, but not sure about October. If you've an extended break then it would be nice to come back a different route - say across the Camargue and then up the A75 and the Tarn Gorges, for a real variety of scenery.
Have you got an ACSI card, too, which also offers discounts on site fees (some can be as little as 10 euros, including electricity)?