I'm not after it myself but is it a 2.7 or a 2.9tonne GVW.
I have an 03 Trafic (same van really) and a 800 litre tank and filling it right up is pushing it very close to the weight limit (mine's a 2.9). If your's is a 2.7, I would certainly suggest keeping the water level a fair way from the top - mind you I'm a fat git anyway. Obviously depends what kit you carry too.
I did check this on a weighbridge once so it's not a rough guess.