Take a look at the Camping Cheque website www.campingcheque.co.uk. There are several sites on there which are open at the end of March/beginning of April. Plein aire des Chenes near Montpellier opens on 21st March, and Camping Neptune, and Camping Champs Blanc near Agde are both open in early April. I'm sure there are others in and around that area.
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