It depends if you’re looking for an app showing wild-camping spots or campsites.
For wild camping, I’ve tried all the apps I could find but always go back to park4night as my go-to app. It’s got all the spots and useful reviews. And at €10/year, it costs effectively nothing.
For campsites, I started with the ACSI app (
https://www.acsi.eu/en/products/acsi-apps/ - The ACSI website is enormously confusing. The ACSI Campsites app is what you want and is just a directory of campsites that they’ve visited. It’s not a special network of campsites, you can’t book sites from the app and you can’t find availability from the app. The app just lets you find campsites and it’s then up to you to call the site or use the campsite’s online booking system to book. They also have a separate “great little campsites” app which is just a subset of the campsites the main Campsites app contains - pointless. Finally, they have a “CampingCard” system that’s completely unrelated to their ACSI Campsites app and that gives you reduced rates at a selection of campsites during low season. You need a separate app for that).
But I’ve now given up on ACSI as Google Maps does the same job for campsites - and it does often a better job as you get more pictures and more reviews. The only benefit of an app would be to be able to search by availability and checkin time and book in one tap. But the ACSI app doesn’t do any of that.