Water
Deciding what to do about water was quite difficult. Legally, the weight of the van has to include any tanks being full. So a large tank under the floor sounds like a good idea, but potentially weighs a lot. As we don't have a shower, we decided to use portable jerry cans under the sink They are 20 litres each - two for water, one for waste. We also just buy 'no name' bottled water for keeping in the fridge.
Using jerry cans makes refilling /emptying easy. Rather than needing a proper grey water drop point, or hoses to connect to water supplies, we can just stand a jerry can under a tap, or tip the waste down a toilet. The sink can also drain through the floor onto the ground when suitable.
A couple of 12 v pumps get the water to the taps. They are pressure switched, so when the tap is off, the pressure in the pipe stops the pump. we've put a switch in the power line for when we leave the van unattended as a leak in the system turns on the pumps and empties the tanks onto the floor.

Two water tanks with quick release pipes to make refilling easier. The dark tank can be left in the sun to warm up. The blue / clear tank is for drinking