1st 2 weeks in Sept you should be ok , the French holidays are usually the first couple of weeks and Italys the last couple of weeks in August.
you could try looking on tourist info websites for the areas you'll be passing through to find where campsites are, we used to contact them and they'd send it all through the post but that was with a couple of months notice.
Do get a phrase book and try to speak a little French , it will be appreciated even if it's as bad as the policeman on 'allo allo' ,just try the basics (hello,goodbye,please,thank you, can i have bread/beer/ect) ignore this if you ever go to Brittany as they'd rather you spoke English as most of them don't speak French but Breton(bit like a French Welsh)