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.
Sink with fold down taps / lid
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
Waste pipe. Vertical pipe through van floor can be blocked with red handle, redirecting water into onboard tank