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 local information centres [look for blue i-SITE signs or visitor information 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.
We recommend downloading the thl Roadtrip app for android or iPhone - you'll get up to date information on where to stay, places of interest on the road, as well as helpful locations such as public bathrooms, dump stations & supermarkets.