Just got back from cycling Roncessvalles to Santiago to Finnisterre. The camino is well marked for the "walking" path, with yellow arrows...but sections difficult to cycle...even with a mountain bike. And I'm sure that during the peak season, with many walking pilgrims, it would be slow going. Even in October, I felt a bit of a nuisance to these walking pilgrims. The roadways are also marked, but not quite as clearly, and some sections had very heavy traffic. I was given a cyclists brochure on arrival/registration for the Camino Frances..seemed to have a lot of details on route and elevation...but was in Spanish only (I'd try contacting Roncessvalles tourism office -- the young man there was very helpful and office well supplied). And while the roadway allows access to the villages along the route, the walking path guides you through them all. There are many beautiful sections for cycling on the walking path, and on the roadways...with a little planning -- yes wish I had brought the Michelin maps -- I would pick the best of each. And just as a measure of this advice...I'm 61 and this was my first unsupported tour, so you might want to scale the "difficulty" accordingly.
Any particular part of Northern France you starting from? I have ridden all over Brittany, Normandy, Pays de la Loire and La Rochelle areas and could upload some gpx files if you interested?
If you starting from Brittany area then St Malo, Dinan and Dinan are must visit places in my opinion. All beautiful places to visit with great food to enjoy :-) Mont Saint Michel in Normandy and Nantes in Pays de la Loire are also great places.