sorry i think thats bad advice.
water weights 1 kilo on an average day with average tempreture
the guy will probably get fined
true, but a 650ltr tank doesn't hold 650ltrs, its very close to 600, check for yourself. Even so,
650ltrs = 650kg
tank = 50kg
driver = 80kg
fuel (diesel approx 0.7kg/ltr) = 40kg
4 poles = 10kg
all other equipment = 20kg (what else you carrying?)
Total = 850Kg and thats assuming a full tank of fuel, a full tank of water and that you are carrying extra gear. Most of the time the van will most prob have half a tank of fuel, also don't forget that the water level is going down all the time. Now if you take into consideration that the 650tanks hold 40kg or so less water than quoted that will more than cover the weight of a securing cage for the tank.
I've had many customers come to me with vans that are overloaded and I have advised them as well as I can but at the same time I've had guys come up to the workshop with systems fitted that are clearly too small, all because some 'supplier' hasn't a clue about payloads etc. The worst I've seen is a Trafic with a 300ltr system, according to the installer that was the largest the trafic payload could cope with. The system was fitted by a very well known company.