I agree the Ibis chain is fine - clean, comfortable and reasonably priced. Personally I wouldn't worry too much about pre-planning hotels; we've driven all over France on spec, never planned a stop in advance, and have never failed to find a hotel room. True, some have been better than others, but none has ever been bad or overpriced. You shouldn't have any trouble at all on the route you're planning. The only publication we've ever used to point the way to a likely hotel is the Rough Guide, and a lot of the time we've just rolled up somewhere that looks pleasant and looked around for a likely looking place to stay. We've only found places full on a handful of occasions - even in the height of summer - and on those few occasions the hotel in question has always recommended somewhere else close by where we've found a bed.