Black Saturday a.k.a. 'Le Grand Depart' is when France - certainly Paris - decamps to the coast (mainly the Med) for a month. The autoroutes get clogged, again especially those radiating from Paris, and given the French style of driving - IAI or Intimidatory, Agressive, and Impatient - it is not surprising that major prangs and subsequent hold-ups result. Having said that if you go around the coast down to the Vendee, apart from the last bit from the Loire onwards where you will meet the Parisienne exudos you should not be too badly hit.
As for a place to stay the municipal Camping St Nicolas at Le-Bec Helluoin just north of Brionne can be highly recommended. Doesn't get too busy, good clean facilities, quiet, and limited children's play area (plus a football pitch.) Its a bit over 200 miles from Dunkerque or from Calais. The village - which is a walk but quite steep - has a couple of eateries, or Brionne has supermarkets and is about 15 minutes drive away. The site sells locally made cider which is highly recommended. Not expensive at less than 12E for car+van+2, but almost no English is spoken by the very pleasant elderly couple who manage the site.