Something along these lines would probably be not too bad:
https://goo.gl/maps/kfChi7Ps5SJ2
They are by no means the exact roads, I just just dragged the blue line around to hit the reasonable bits, having set Google maps to avoid motorways and tolls. For instance, you could move the line upwards to cross deeper into the Morvan, exiting eastwards towards Dijon and Belfort. If nothing else, it inevitably shows how difficult (or easy, depending on the effort people are prepared to make) to offer up routes of one thousand or more miles, along with things to see on the way.
Total distance Le Mans > Oradour > Belforth > Calais is 1,100 miles. You have Monday through Sunday to do the journey, which is seven full days, so plenty of time. Take off one day to look at Oradour and a full day in Belforth and (maybe) half day to catch your return train in Calais (assuming a mid-afternoon Sunday crossing) leaves 4.5 days or an average of 250 miles a day on French N and D roads, which is quite doable. That leaves time for a morning coffee, lunchtime lunch, an afternoon coffee and whatever flower pressing you need to do, all without breaking your butts.
In planning (there's a dreadful word for all seat of the pants Adventure bikermates) 1000 mile jaunts across France, a great help is the excellent Michelin 726 map, which looks like:
http://www.themapcentre.com/michelin-route-planning-france-2017-6955-p.asp
It's designed specifically for help in planning routes such as yours across a big country. It has all the motorways, which you don't have to take. More importantly, it has all the main roads and the suggested 'holiday' Bis routes coloured green on the map - you can see a short segment in the picture below, running west from Dreux in the bottom left hand corner - (which have their own distinctive road signs, too) but without all the clutter of some very detailed maps. Similarly, any town with a green box around it not only appears in Michelin's excellent green guides but will at some point appear on all local road signs, too. Use it to get a good feel for the direction, roads to take and fill in any fine detail with other Michelin maps. The useful thing about the other maps from Michelin is that they can be found in the same scale, so France always appears at a uniform size. Maps are cheap, last a good number of years and unlike your GPS will not let you down. Buy and use with confidence.
Now sit down with your mates and plan (or not) your holiday.
Richard