Hi Nigel
If you take the N road, rather than the autoroute there are several sites along that road, starting at Nevers, where there is a municipal site, then Chatel de Neuve where there are two sites, St Pourcain sur Sioule, again a choice of two sites, Grannat where there's a municipal site, and just south of Clermont Ferrand is the muncipal site at Issoire. Obviously you're travelling in peak season, so it may be necessary to at least check availability before going out of your way. If you're doing the autoroute itself, north of Clermont Ferrand, then this website http://users.telenet.be/leo.huybrechts/camp1.htm
may help you find other sites along your changed route! It lists ALL sites within 15 km of the major routes through France - just click on the flag, select your route, and then the sites are listed from north to south, so it's very easy to find one wherever you are.