I have camped quite a bit in Europe with German friends who own an RV. I have lived in Europe as well, and I can verify that all of the comments made here about costs, etc. are true.
I would consider renting some sort of cabin at a campground as well. I have done this and have been very happy with the result. All but the most bare bones campgrounds have these, and they vary from just basic sheds to full-fledged homes with kitchens, bathrooms, and satellite TV's. Prices depend on season and amenities. Campgrounds vary widely as well from economical municipal campgrounds to five-star resorts. With the price of hostels really out of sight, I bet you could do this for two people at no greater expense.
France is the best place to camp; I can highly recommend it.
Send me a private message if you want more detail.