Provided the campervan you have rented has toilet facilities on board you are allowed to 'freedom camp' in designated sites. Please obey local signage and regulations, and ask at the local information centre [look for blue i-SITE signs in each town] for suitable places to stay. Please do be aware that you may be fined for 'freedom camping' in non-designated areas.
To fully charge your campervan's auxiliary [house] battery you will need to plug the vehicle into 240V mains power regularly. We recommend you stay in holidays parks to charge the battery at a minimum every second night.