alanwilson is right - the best way to solve this problem is to look under the bonnet, or in the door close, for the vehicle plate. This will give the various figures you need. It should list the GVW (gross vehicle weight) and the GTW (gross train weight - including a trailer)
Load up your van with all the equipment you are going to carry, including the empty pure water tank. Fill the fuel tank.
Take the van to the nearest public weighbridge (listed in Yellow Pages) sit in the van, with a passenger if you are going to have one as a rule, then weigh the van. The difference between the weight recorded by the weighbridge and the GVW is the maximum amount of water you can put in it.
If you want to calculate before you install the tank, the suppliers should be able to give you accurate dry weights of any size of tank.