All vehicles require the additive AdBlue to reduce nitrogen oxide emissions.
The AdBlue tank provided for this purpose (separate from the diesel tank) is filled before your pickup. The range varies depending on the vehicle model, consumption and driving style.
If the AdBlue tank runs empty while you are on the road, you are responsible for refilling it, because AdBlue is one of the operating fluids in your campervan, just like diesel or wiper fluid.
AdBlue can be easily purchased at almost any service station.