look at the gross vehicle weight that amount is the important one, the van cannot weigh more than that,
best thing to do would be to use a public weigh bridge check the weight of the van empty so you know how much you can put in it
That's right
Somewhere on the van will be a plate with the weight figures on it. GVW means Gross Vehicle Weight (It may not actually have the letters GVW but you can work out what they mean - sometimes there's a second figure which is higher - this is the Gross Train Weight which means the most the vehicle can weigh in combination with a trailer.)
Load up the van with everything you are going to need/carry including fuel, driver and any regular passengers but leave the water tank EMPTY
Take it to you friendly local weighbridge (either the local tip or scrap metal merchant) and get it weighed WITH DRIVER /PASSENGER(s) SITTING IN IT
The difference between the weight you get and the GVW is the amount of water you can put in it.
You will get a certificate showing the recorded weight - keep this in the van in case you ever get stopped.