I haven't toured along the Canal du Midi, but I have toured extensively near it in SW France. My touring in the region has usually been in either June or September, and I've never had any problem finding a room in those months. For several reasons (crowds, hot weather, flight cost), I suggest you try and go in those months instead of July or August. It's pretty easy to find inexpensive two star hotels throughout France, in addition to chambres d'hote (B&B) and the more primitive and cheaper gites.