You won't know unless the day you get it and put it on some scales, this will show you how much you have left on your axles (GTM) and how much you have on your tow ball, add the tow ball and axle weight together and subtract it from the plated ATM and that will give you the amount you have left before you pop the scales on your ATM for van............................ the tare weight marked on the plate unless you are very lucky won't be correct and shouldn't even be bothered with.... ATM and GTM are the important ones......
Most vans state either a 400 or 600kg load, which doesn't include water, gas and any extras that is fitted by the dealer, some have found while their van is under the ATM a fridge full of food puts them over................ I have just replaced the hitch on mine and its 6kg heavier than the basic one I replaced, every little bit adds, nothing to worry about just something to keep in mind....