The only safe way to calculate total weight of the van is to check the 'plate' - this will be either under the bonnet or in the door closure and shows the GVW and GTW. The first is the total that the vehicle can weigh with everything in it (Gross Vehicle Weight) and the second is the most it can weigh including a trailer (Gross Train Weight)
Fill the fuel tank. Put on everything you will need to carry (Ladders/equipment/tools/spares etc etc.) Make sure your water tank is empty. Get your usual passenger to sit in, drive to the nearest public weighbridge (Yellow Pages) and weigh it with you both sitting in it.
The difference between the weight you record, and the GVW is the amount of water you can legally carry.