Not every1 wants their 15 minutes of fame....

Alfie, so your "guess" that van is not overweight ?
Well I concede that may be the case, but only if you are under the average weight of 85 kilos in weight, and only have quarter of tank of fuel, you should be ok with a window vac as your spare tyre is missing !!!
Please bear in mind that the RO weights more once used and holds a fair bit of water so the filters, di vessel and hose reel adding even more weight to the water in the actual tank.
However, regardless of our guesses, the only sure way is to have the vehicle weighed.
My observations are, it's a very poor "professionally" fitted setup. Just my opinion
Darran