If you are travelling with a tent and not with a trailer or campervan it won't be a problem. At most camping sites there is always space for a tent. With a campervan or trailer you'll have to be a bit flexible, maybe driving 20 km on if everything is full.
But if you use the AA camping guide you should be able to call the camping sites in advance and ask whether they are full, there are usually 2-5 within a 50 km radius.
I drove across France last year and it cost me about £70 in tolls. Buying anything on the motorway is expensive so I came off to get fuel and a bite to eat. Toll roads all along the south of France into Italy too. Driving the Alps can be hard work even in summer as it is very slow.
Do you really want to do this with a wife and baby daughter when you will spend a good part of 4 days driving (and ferries)? Have you considered maybe a Ryanair flight to Marseilles and pick up a car there, either using it as a base or travelling around? Or maybe even a coach tour?
Done a fair bit of Europe on my motorbike - as a kid all our holidays were camping in Europe - you'll have a wonderful time - when in France, unless you are in a rush, I would try and avoid the motorways - this way you get to see some wonderful towns and villages en-route. I'm sure whichever route you choose you'll have a great time.
One bit of advice - for France in particular, if avoiding motorways - fill up with fuel when you can !! Many towns still observe an afternoon shut down of all shops and garages and there's nothing worse than running on fumes with the next town about 80k's away !!! As you are entering towns you will usually see billboards advertising which hypermarket or supermarket is in the town with directions on where to find it, they usually have fuel.
"Camping Municipal" are the sort of local council run campsites and are usually great value for money if you need to camp in or near to a town - many have great facilities. Alternatively (if you don't feel like pitching the tent) there is a great network of farm B&B's (auberge) at reasonable prices usually.
Don't forget your High Vis vests - leagl requirement !!!