I use the CamperContact app, which is around £5 I think. Park4Night and Search4Sites are alternatives but I find CamperContact suits me. Occasionally, if there's a specific place I want to stop at I will look at the websites of the others.
But personal preference comes into it and CamperContact doesn't suit everyone.
The Android version of Camperstop has a lot one star reviews. Can't speak for the iOS version, assuming there is one.
One of the other options, mentioned many times here, is Campercontact.
This is a Dutch app but works in English with no problems.
It will list most of the campsites and Aires you may be looking for.
From memory it costs about £6 per year.
I think it has a 7day free trial ?
Seems to need plenty of memory, so don't try it on an underpowered or phone that is short of memory.
Works very well for me.
I assume Camperstop is similar.
You will find, I expect costs will be similar as well.
One free alternative website and app is searchforsites.
This lists many stops in all sorts of places but just check they are still there and legal before you go.
CamperContact does use quite a bit of memory if you download all the images, but you don't have to do this. Because I have a MiFi running all the time in the van my tablet (which doesn't have a huge amount of memory) is permanently on line. When I open an entry in CamperContact it will download the images for that location only.
It would be better if you could select only to download the images for specific countries but this isn't currently available. I also have it on my phone which has plenty of memory so this has all the images downloaded.
Time was we took "All the Aires..." Park 4 night, Camperstop and Camper Contact.
All the Aires changes once a year and is quite expensive if you want France and Spain/Portugal.
Park4night is good but inclined to crash from time to time.
We only used the Camperstop book and as we have limited payload, that had to go.
Camper Contact for around £5 per year is up to date, easy to use and I can stick to a single format when using coordinates.
We also use it with a Mi-Fi so not too much to download.