Necessary to prebook?

September is pretty major tourist time in France; it is probably the prettiest month to travel. If you want to meander and nor pre-book I would book the first couple of nights and have a list of places so you can pre-book the day before. Years ago one May we were driving in the south of France and I had a list and it was before we had mobile internet and we went to six hotels without finding a room; it took a lot of time and we were actually getting worried we might sleep in the car when finally we found a place. It was like prison cells -- the bed was a poured concrete platform with a foam mattress but we were grateful to find it. So have a general route planned and then you can adjust as you go but reserve the day before. We have done that driving from Paris to the Dordogne in September. I have always had better luck reserving ahead than trying to get things on the fly. Maybe you will have better luck.

Thanks for your thoughts. We have travelled a couple of times without prebooking and it's been a gamble.

I almost always get really nice quaint charming lovely hotels when I book ahead and my track record on the road is laughable. I swear we stayed at the Bates hotel one time in southern France on our way to Florence and we were the only guests that night. And then there was the time trucks were rolling underneath our bed all night when we had a room that bridged a highway. You may be more gifted but I am really bad at this.

It would def be easier to do with a car and internet connection.

The car will give you more options if everything but the flea traps are booked, and it's nice being able to pop onto Expedia or booking.com or airbnb and book a place for that evening or the next day.

Depending on what you want to stay in (B&B? Hotel?) booking at the check in desk on the day of may be more expensive than booking ahead. I wouldn't do it without a car unless you are ready to pay $$$ or stay in a cheap hostel, or to drag your bags around trying to find somewhere to stay.