I want to highly recommend the Application Pro Mobil for explorer in Europe who go by car, caravan or “Bulli” like I mostly do. It shows you camping sites wherever you want to stay in Europe. It reveals not only the big and expensive ones - full of families with annoying kids who stay there the whole summer, but also quiet places where there is merely space for a few big cars and which are sometimes limited to one or two nights.
Those are practical if you are passing through. I have been at a camping site in Bibao (Spain) where you could stay for two nights. Restrooms, internet, power and water were provided for 15€ a day. A bus station was directly at the front of the area and the view was absolutely amazing. Take a look!
In some countries in Europe it is forbidden to stay the night at a normal parking place – the app includes areas where it is permitted in those countries. Those are usually limited as well and do not provide anything. At least you can be sure not to get a ticket.
You can enter your position or the app can find it on its own if you permit the GPS to work on your mobile phone and then it reveals every camping site in the vicinity. You can limit the radius and it is divided by color into camping sites that cost less than 5€ (green) or more (red).