You switch the 12 V pump off when on mains water. This applies to all vans. If you leave the pump on and the town water is lower in pressure you will pump from the tank, many have reported this happening. The two water systems flow through one way (non return) valves. This stops the mains feeding to the tank and also stops the 12 V pump emptying the water tank through the mains water hose inlet.

Scroll to the bottom of this link to the heading "4. Adding mains pressure and hot water." This shows how most van water systems are plumbed.