Camping off the grid, meaning not plugged in to an electrical socket, you need a way to recharge your batteries inside the living space for everything runs off of those batteries which are separate from the car battery that runs the automobile.
The most common way is to get power from the sun - solar power through solar panels. This panel is mounted so that the owner can tilt it towards the sun for maximum efficiency. The sun's rays are hitting the panel perpendicular to the panel surface rather than on a slant.
Then there are flexible solar panels for those who do not have a flat roof surface to mount a hard panel. I understand that these panels are not as efficient which is probably why this owner has four of them out in the sun. Being flexible you can lay them on your irregular roof and also they are easy to store away when traveling.
This is what is commonly known as a suitcase solar panel. It folds in half and comes in a case to store it in when traveling. They work as good as the one mounted permanently on the roof but as you can imagine takes up a lot of room inside when stored away.
Lastly there is wind power to generate electricity for charging the batteries. They work good but are very noisy and must be really anchored down solidly as you can see here. They need to be taken down when traveling. Not a good idea barreling down the road with this thing up.