The power cable plugs into the outside of the bus, left rear, and connects to the power supply at the campsite. You will know the car is charging when a plug symbol lights up in the camping display.
Do not forget to unplug the power cable when driving off! To do this, push down the blue lever to the left of the cable.
Without power connection you are self-sufficient with electricity for about 2 days. Then you have to drive again for some time to recharge the battery.
You can see a video explanation here (minute 4:55):