I have the Nissan Primastar version of the same van.
Mine is the short wheelbase one, max load is just over the ton, so although you can fit a 1000L tank in there you will be over the limit.
Ladderman is right, a 650L tank is the safe option, you could go for the 850L tank, but again, once fully loaded with water, diesel, all your other work gear and you sat behind the wheel you will once again be well overweight :
1000 litres of water will weigh 1000 kilos. 1 litre=1 kilo
If it is an IBC tank then it has to be in a frame, and ideally a custom made one, add the weight of that to the total and things are getting heavy!
In my own van I have a 650 litre tank, and as a one man band working all day long doing tops and bottoms WFP I have very
rarely used a full tank of water, and I work with a very high flow rate too.
From your post it sounds like there are at least 2 of you, so with 650L you should be ok, I reckon on most full days I'm using around 300l of water a day, and as I say, I have my pump going flat out all the time.
Don't risk going for the big tank and putting the van over weight, get pulled over for checks, or be unlucky enough to be involved in an accident and you'll be in the do-do
Ian