Rather than Camping Cheques , buy the ACSI book which comes with a card that gives you fixed prices at 100's of Continental campsites mainly French. It gives price per night per campsite, I think this year it was 11/13/15 euro and it for use in low/mid season not high season.
CC's you buy upfront so you are paying for campsite per night in advance. ACSI card you buy the discount card for about £15€ & then pay for campsite at discounted price as you go. Both will give you lower campsite cost than turning up & paying but can only be used out of high season ie not usually Jul/Aug.
How much you save depends on campsite, some charge much the same out of high season than CC or ACSI so best advantage is usually on the more expensive campsites.
You can only use campsites that are in the respective schemes though & a guide book is available for either or just refer to websites.
Read their websites. Google Camping Cheques & ACSI.