I read a post on here about the aires on the motorways south of calais this is extremly dangerous If you have done this without problems rhen you were hust lucky. This is open season for thieves
If you want safe free parking if you arrive late ferry in Calais then drive 20miles or so in the direction you are heading then drive into a small town & find a supermarket carpark. Thats what I do.
If you want to park after coming off Eurotunnel on French side you can park at the services within the Eurotunnel complex which is secure enough.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.