Large commercial = high flow rate, large usually means lots of big windows, back pack totally inappropriate for work of this nature.
I suppose that with a variflow attached you will be able to cut the amount of water you use to a minimum, but you'll struggle to cope with just 500l unless you can fill up.
If you are working 30 miles from home, what is the tds of the tap water in the area's you are working?
If low enough, as has been suggested you can work DI only and simply find a tap to top up your tank.
But for two men, be sure you get the larger Vivaro/traffic/primastar van, (should you go for a larger van of course) my own (a primastar) has a payload of just over 1000k, I have a 650l tank, were I to fit an 850l one in there, once a full tank of fuel, my own weight and the weight of all my equipment is taken into consideration I would almost certainly be overweight.
Ian