Noooo! Seriously. Don't bother with campsites with a Motorhome in France. There's no need. It's all about aires and wild camping in France. France is setup for motorhoming. It's easily as good as Germany.
For context, we've had a Motorhome since 2012 and been at least once, if not three times a year in ours since. Including a 5 month long trip during my mat leave (although we also got Portugal and Spain in too). I reckon we've clocked up at least two years worth of over nights in France. We've stayed on a campsite twice!!!!! Both municipal. It's nice because it was cheaper and more spacious (very spacious) than the aire. The other time. Gorgeous spot, 5€ a night.
I'd suggest downloading Park4Night app. I used to have the Vicarious media aire books - but everything is on the app. Then you get the benefit of reading up to date recent reviews, these always help me decide!
If you don't know what an aire is, don't confuse it with the French motorway aires. Do not overnight in those - unless you fancy risking being robbed.
An aire is a parking lot designated for Motorhomes to stay at overnight. Basically the same as a German stellplatz which I assume you'd be familiar with? They usually have free fresh water, a drain for grey waste and toilet disposal point. Some have electric. But that is irrelevant for us as we have a solar panel. Some aires you pay a small amount to stay over. Many are free. For some you'll need to pay to use the water. Avoid the camping car park ones - expensive! Most towns, villages and cities have them. Some are restricted to 24 hour stays, others 72. Some not restricted.
Park4Night will also show you spots for day time parking suitable for your van. It'll show you tolerated spots for overnighting too. It shows farm stays, some campsites (but why bother) and France passion. Legally you can wolf camp in France, although there are restrictions in some areas. But it'll be quite clear when you are there where these are.
France is a Mecca for Motorhomes. You'll see zillions!
Our van is 7m 30 for context. Never had issues not fitting anywhere.
The west coast is Brittany. It's lovely but maybe not in the school hols. It'd be teeming. We do Brittany over Easter or October when it's quiet! The Normandy coast is spectacular and people tend to make a bee line for Brittany. Further down from Brittany, like La Rochelle, Il de Rey and to coast down to Bordeaux is uninspiring and busy.
There are a wealth of rivers and large lakes which I would pick over the coast to escape the masses during the hols. The Lot and Lac de Vassiviere are two faves.
I would suggest joining some Motorhome full time France groups on Facebook. Then posting your specific requirements - but be very specific! Eg what works for your family won't necessarily work for ours. We pick areas for wild swimming, kayaking, cycling and walking. And. Most importantly to get away from it all. The traffic in those groups on Facebook is high so I'm sure you'd get helpful suggestions as long as you are very specific about what you want.
Enjoy! You won't be disappointed! If it helps - we avoid pèage (unless to skip round a huge city). The D and N roads in France are awesome - not as fast as Germany. But good. We always say it's about the journey not the destination! We also tend not to plan more than a night or so ahead. That way if the weather isn't great, we'll head for where it is. For example we'd wanted to go back to Brittany for a few years, but only recently made it. The weather had always been bad so we went where it was good!