The Michelin 'Camping France'
guide has maps covering the whole country and shows towns with 'at least one selected site open all year'by underlining them. You do need then to double-check with the main entry, because 'all year' may not necessarily mean 365 days per year. Otherwise, the Alan Rogers France Guide has a list of all-year sites, but this only gives about 40 in total. Most other guides give opening dates, but it can be a bit tedious finding those that are open at the times you need. If in doubt, always phone, as advertised opening dates can change at short notice.