Not sure but do chech with Caravan and Motorhome club and Camping and Caravaning Club, they offer discounts on many routes.
Also check Harwich to Hook van Holland. Yes you will have another 250 miles or so to drive but it is a shorter crossing and no cabin required for day time sailings so much cheaper. ( You can stay in the Harwich ferry car park free of charge. Quiet, no facilities except WiFi. Toilets for a couple of hours before ferry goes. )
I don't use Dover to Calais or Dunkirk. This is cheaper still but you have the Dartford crossing and then have to drive a long way north.
Hull Rotterdam is also possible but I've never used it.