If my memory serves me, you've been using a backpack so you have learnt how to minimise water use.
My son in law uses anything between 250 litres and 450 litre a day depending on what area he is doing. He only does residential. He works quickly and runs his flow controller on about 2 LPM. (2LPM is only around 3.75 hours running time and that will use his 450 litres.)
I think you also have some work which is a bit of a distance away, so you will need a full tank for that. Even if you can top up on route, this option may not always be available in the future.
As I say, my experience is to buy the right van to begin with. I wouldn't reduce the size of your tank to accomodate the van you like. I made that mistake when I bought a WFP trailer. It had 3 x 125 litre tanks and those weren't sufficient to last the day residential. I carried a further 100 litres in 25 litre containers in the van (Suzuki Carry) and usually ended up using all that as well before 3.30pm.
I now have a Citroen Relay SWB with a 650 litre tank - plenty enough for the day and no, my Relay doesn't use much more diesel than the Suzuki uses petrol.