Just bringing this back to the top again. We've thought about this, too, so hoping someone might reply! We've also thought about storage in that area, and then having the freedom to tour as well - and haven't come to a decision about which is best (although storage is certainly cheaper). I suppose it depends how much time you'll actually spend there. We haven't really investigated the 'permanent pitch' option, as we have to stick to just 'holidays' rather than permanent stays in France, but have looked at some storage sites, some of which will tow your van to site and sort things out before you arrive, which sounds a good compromise to me.
Not that many sites are actually open all year - and those which are tend to be just mobile homes on site, with touring pitches definitely just kept for touring. Serignan Plage, however, obviously does have some well-established permanent pitches for ordinary caravans, which have become real home from homes - but the site only opens for quite a short season.
We've also investigated buying a 'Terrain de Loisirs' which often are quite cheap, and you can site a caravan, and use it for a proportion of the year for 'leisure' - depends on the local Mairie and bye-laws as to what proportion this is. However, again some limit you to between April and September!!