Just back from France after a months travelling around.
We used Campingcarpark.com (get app on Google Play).
Great bit of kit tbh, we used 4 of their sites, whilst en-route.
Tells you how many spaces available, allows you to just 'rock up' if spce avail, and you can filter what you need, ie Loos/Shower, basic stopover etc etc.
Eur5 for a lifetime membership, or Eur 30 for the privilege pack (this allows you to book in advance, rather than just turn up, also allows you to come and go from the site & not loose your space).
We opted for sites with facilities, all were clean, spotless, quiet, and incredible VFM (+- Eur14 a night, with hook up & sanitary).
Perfect for 1 - 3 nights if your travelling around but doing some visiting in an area as well.
Don't dismiss 'Aires', as whilst basic, are superb if your needing an o/nite stop.
We didn't 'book' anything, just worked out where we wanted to be, checked the app for space, and rocked up.