Your California is equipped with 2 USB ports and 1 socket. This can easily be found behind the driver's seat, built into the kitchen cabinet.
You might find some more USB Ports, on the top up bed for instance.
The USB Ports can be used if you have enough power on the leisure battery but the socket only works if the van is plugged with the electrical cable.
To reach out the USB ports behind the driver´s seat just rotate this wheel (first photo below) until you get the seat like this (second photo below).
Other relevant articles:
How to charge the leisure battery