It depends on the festival, but when they have a campsite available or nearby then yes you can camp at that festival. Have a look at these awesome music festivals in Europe of which most of them offer camping spaces. Just make sure to check whether you need to purchase a separate campervan / camping ticket or if it's already included in your festival ticket.
A Spaceship is perfect for festivals as they are classified as a (camping) car and not as big motorhomes. They're easy to drive around and it'll save you heaps of money. Plus, you'll have your drinks and snacks in the fridge or storage, a gas cooker and everything to refuel yourself before partying hard again. Oh and all your belongings are stored safe and secure in a Spaceship.