Have a look on What Van?'s website - they give the payload for all vans.
Payload is what you can put in when the van's got half a tank of fuel, usual tools and spare wheel etc.
Payload includes driver and passenger.
Best way is to take the van to the nearest weighbridge. Weigh it with yourself and passenger and all the gear you usually carry. The registration document or owners handbook will give you the GVM - "gross vehicle mass" that is the maximum the vehicle is allowed to weigh. The difference between the weight on the weighbridge and the GVM is the spare you've got for other gear (RO if it's on board, tank and water, etc etc.)
Cheers,
Ian