Lots of vans have more than 1 water tank, often 2 or 3. But the way they are plumbed doesn't make the most of them.

If you are travelling to areas that have limited water supplies, you are going to need to separate your drinking (potable) water from your non-drinking (non-potable) water. 2 tanks are necessary, but they can't use the same pump or plumbing - so 2 pumps are required. Most vans do NOT have a 2nd pump.

The best configuration is to have your drinking/potable water tank connected through its own pump, to a separate filtered drinking water tap at the kitchen. The rest of the plumbing - shower, sink, external and your hot water system - can all run from the non-potable side. 

To get a bit more advanced, you can also fit a transfer circuit where you can pump from your potable tank into your non-potable tank, which can help in certain situations. You generally don't want it to go the other way for risk of contaminating your drinking water system.

Regularly use tank cleaning tablets to keep everything in your plumbing healthy!