There is a campground that I have stayed at right outside of Blois. Blois is a beautiful city on the river. If you are lucky, the chocolate factory will be working and the air will smell of chocolate. At least there used to be a Pulain chocolate factory there; I assume it's still operational.
So check out the campgrounds in the area. I'm sorry I cannot remember the name of it. It's likely to fill up with the Tour coming through, so check soon.
There is also a youth hostel in Blois with 48 places, 4,5 km away (18 rue de l'Hotel Pasquier, 54782721) and another one with 35 places and 10 tent places, 10 km away (Levée de la Loire, 38446131). But if TdF passes there are no rules: ask someone who has anough place for a tent, it will be okay, just be friendly.