The distance is just over 500 KM or about 320 miles or so, about 300 miles of the route is motorway.
French motorways from Calais down towards Rouen is not like English frantic motorway traffic levels.
For your fisrt run with the caravan I would aclimatise to the French Road and being on the "wrong side" on the easy to use motorway rather than get into junctions and roundabouts etc firts time off. Take the motorway out of Calais and head south and plan a detou stop into Le Havre say or some other larger town / ty on route to experience the A and B roads.
French motorway driving with a caravan is easy , most brits are surprised by how little traffic they see compared to back home.
http://www.viamichelin.co.uk
Enter Calais and StMalo and set to car and caravan!
Bon Route