The van needs to be plugged into a mains supply at a campsite to charge. Follow the next steps:
Note: Your California also charges while driving, but not as quickly or efficiently as with the cable at the campsite.
1. Grab your cable from the trunk and plug the three-phase socket into the hatch on the driver´s side.
2. Plug the other end into the campsite socket.
The majority of the campsites work with the normal socket. If you are asked for the three-phase socket, simply disconnect the converter. (second photo)
3. After the battery is charged, to unplug the three-phase socket into the hatch on the driver´s side, please press the blue lock like on the photo below. If yours doesn't have this, just unplug carefully.
Other relevant articles:
How to monitor the leisure battery level