We have always found France to be quite easy. Buy the Aires books you will be suprised at the location of most of them, by the sea or lakes and near to towns.
A couple we have just stayed on that are next to the sea are Berck Sur Mer and Forte Mahon Sur Plage. These were both paid Aires, with fresh water, waste water and toilet dump! The first being 6 euros, the second being 8 euros.
Lots of families were at these two, they took the kids to the beach during the day and they are not too far from Calais. They are on the icampsites data base if you have that or in the Aires books.
In France, if one site is full, there is another close by. If it is campsites that you do want, then you may want to try the Municiple sites, they are normally sign posted in towns and not a million miles from Aires.
As for ferry or tunnel, we have only ever used the ferry , time to stretch your legs, but will be considering the tunnel next time as we took the dog for the first time this year and unusally for him, he was very quite when we returned to the van on our return, mind you the sea was very rough.
Enjoy your trip and your motorhome