Although you can stop at most French ferry ports, my advice would always be to buy a copy of the Michelin Campsite Guide and see if there is a proper site close by. You may prefer to start your holiday in a car park with noise all around, feeling nervous every time a car pulls up or a gang of yobs walks past, a refridgerated lorry leaving its motor running just a few yards away but it's not for me. Even the most basic site will be on grass, will be reasonably quiet and, most important of all, safe. If you arrive late you may have to stay in the late arrivals enclosure. You can still get out, go for a walk and leave your van feeling reasonably sure it will be there on your return.