In reply to J1234:
Yes, they're the Municipal campsites. I think at one time every big village / town was either encouraged or required to provide a campsite. They would often be on the school or village playing fields and would use the changing room facilities, etc and be open solely during the school summer holidays.
Many have become more established with students as site wardens but the philosophy of them being a 'service' to the public rather than a money making concern continues. They often have special rates for cyclists and traditionally would never turn you away however full.
The old yellow covered 1:100000 Michelin maps of France used to have a tiny black solid triangle next to a place name if there was a municipal campsite in the area and that was all the planning you needed to tour France in summer. Simply joined the triangles together!