Good points.
Note: my axle ratio example would already be factored in to the GCWR.
I'd suggest getting the numbers for the particular van you are interested in if you have a specific towing requirement in mind. The info on the manufacturers websites is for a generic model and won't included optional equipment for example. Optional equipment, fuel, water, propane, cargo, passengers, clothing, gear, food etc. all would be added to the UVW and the subtracted from the GCWR.
If anyone here has weighed a packed and loaded-for-travel newer Sprinter based Class B please post what it weighed.