Calais to Boulogne N1 to Abbeville A 28 to Rouen N 154 to Chartres and N10 to Tours. Doesn't take much longer than motorway, much of the earlier section is basically motorway but without the cost anyway. Motorways are soooo expensive. We did it a few weeks ago. Don't have my timings for the way down but on the way back we left our campsite which was near Amboise (Between Tours and Blois) at 9 and got to our hotel near Boulogne for 3 with a good lunch break on the way.
It is very good and in a more direct line than the motorway. Some sections are a little frustrating when you go through towns but mostly it is easy going, also more relaxed than motorways and makes for nicer stops as you just pull up ina village or town somewhere.