Hi there,
Dunkerque was a bit rough the last time i was there, however there is a fantastic falafel bar in town!
I would try
this site, they're recommended. You should have plenty of time for riding, the 940 south from Calais is a nice road with plenty of seaside towns to aim at.
Have a good one and don't get too hung up on accomodation in France - there's lots!!!
Regards,
