Just came back from five weeks in different parts of popular walking routes in France, the Alps, High Provence, Chemin Saint Gulhem le DĂ©sert 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.