SMA65,
from Calais the A28 now goes direclty into Rouen, so its dual carriageway or motorway all the way there, navigating through Rouen is now not so difficult.............
just continue straight on from the end of the A28 towards the centre of Rouen (still on dual carriageway) cross the river Seine, once over the bridge turn left, keep following the dual carriageway through all the "Zone industrial" eventually you'll see the sign for the A13 & eh viola!!
See this link for travelling times, add about 1/2 hour for St Jean de Monts.http://www.practicalcaravan.com/newforums/fm_messages.asp?FO=7&FM=287954
For the best directions use www.viamichelin.com it even has the location of all the camera's & what limit they're set at.
GeorgeB