Just came back from five weeks in different parts of popular walking routes in France, the Alps, High Provence, Chemin Saint Gulhem le Désert and the Le Puy route. Yes the French book ahead but I just booked one day ahead, I got the gite I was staying in to do this for me and sometimes the local tourist office. I had only problems in an area of Provence (which is not a camino route). The worst case scenario is that you might have to walk less or more in a day to find accom, but that will happen only rarely. I have been told by gite gite owners that some French book in January for summer booking an entire trip in advance but as a result they said they have a lot of cancellations and no shows.
Three years ago I walked the Le Puy route and never booked ahead and had no problems but on that trip I had a companion that I could share a room with if gites were full, this happened only twice and because we did not arrive till late in evening. When I am alone I will book one day in advance.
Don't tie yourself to a crazy advance schedule by booing all in advance. As you can see from my blog and profile I have walked a lot in France in the last few years and the most I ever booked ahead was one night, apart from my first night which I book about two weeks before I fly out.
I once, because I had no map came down the wrong side of the Cevennes and ended up 50km from my booked gite, I hitched and got there for dinner (not on a camino route). Just enjoy your trip.