Hi John, I have had a Scudo multijet 90 for over 3 years now it's a brilliant van. The short wheelbase payload is 1000KG & I have a 400 litre tank in it. I don't know if you are aware but a litre of water weighs 1 kilo so you can work it out from that.
If the long wheelbase payload is 1200KG then a 1000 litre tank with the weight of extra,s & fittings should be well within the payload limit.
Regards Rickster