France is massive. I would suggest you take as much time as you can. A month is a nice break but you wont even scratch the surface. There is tons of info and pics on my blog at
Hank the Tank Motorhome website most of it is France.
If your going April and want good weather I would suggest getting down as far as the Dordogne and beyond. Last spring in April we did the Dordogne, Pyrenees from East to West along the middle on the French side then back up to the Dordogne and Lot vally and home via Brittany. Took us two months.
The Dordogne is lovely but we love the mountains and we travelled from Perpignon to Biarritz along the Pyrenees which was simply superb. Places like Cauterets and Gavarnie were simply stunning and it was 25 degrees and sunny in the valley but up at 7000ft people were still skiing and snowboarding.
On the other side if you go down as far as Provence in the Provence Alps you could pretty much draw a line from the Verdon Gorges heading slowly west towards the Dordogne to include L'Ardeche, Averyone region, Lot and Dordogne and its all fantastic. We did all that on this years trip. Aires will be quiet and there are some crackers. Personally Im not a big fan of the Med coast and I am yet to come across anywhere I would rush back to. Normandy and Brittany however are lovely but you cant guarentee the weather.