Camping Olivet at Orleans - provided you are going that way, and I don't know whether you are. You could be using a different route altogether via Le Mans and Tours. Sorry.
If you're going via Orleans you might wan to travel further depending on your arrival time in Calais. There are a few sites further on - e.g. Nolan le Fuzzier, Salbris, Vierzon. If by Le Mans and Tours then Beaumont sur Sartre north of Le Mans or The site at St Avertin immediately south of Tours.
As Berriow says, it very much depends on your route, but if you're doing the normal 'skirt around Paris' on either the west or east sides, then you'll generally continue via Orleans, so from Orleans south are Berriow's suggestion, then there are also sites at Nouan Les Fuselier (Camping Grand Sologne), Salbris (Camping Sologne), Vierzon (Camping Bellon), and at Vatan (La Rouelle Aux Loups). All of these are near to the likely route (A10, A71 or D2020) and none need to be booked in advance.
We have done Calais to the Dordogne, with an overnight stop over at La Rochat Belle Isle, at Chateauroux which is even further south.
We find that sometimes journeys can have very different timings, so we never book overnight stops - just find somewhere when we're ready to stop.
Yes, this one. It's cheap & there is always space, so no need to book. Carrefour supermkt almost next door & only 5mins off A20 motorway. Website don't work but it's open April-Oct.
Don't forget to leave a review of the French and other European campsites you have visited!
Well, Ridgecroft, there you have ten suggestions already, and if truth were told there's scarcely a town or village on the way to the Dordogne which doesn't have a campsite. We usually try and decide by lunch time on the day in question, but quite often change our minds mid-afternoon. Don't over-plan !
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.