Yes,
I just put the hose in the top and away i go, its unlikely you will overflow unless you have a really powerfull hose, but you will gauge it before you put it in the tank and the tanks are so big you get plenty of time before it would overflow, just check levels after 10 mins and crack on.
Although you could make a system up as some others have, i just think its ott.
Water goes in, water comes out, what else is there?
The most likely thing is you will use more water than comes in, but thats why if youve got a 1/5 starting point you are ok, you will most likely get through an average job without having to stop, where you might stop 4/5 times normally.
seriously though dont drive around with too much water in, your van will struggle and your fuel economy will halve (Its not just the weight its the sloshing around).
The most important thing is to make sure you have some very heavy duty straps (cargo/ratchet type?) to secure the tank to the van floor, they should be wrapped round the tank (not just through a couple of bars of the frame) with the emphasis being that the tank cannot slide to the front of the van if you were in an accident. I dread to think the forces involved even at 30 mph (>10 tonns?) should you be in an accident. It would almost certainly crush you.
Ive had a quick look and these guys seem to be decent enough, you need a couple of sets. if anyone knows of heavier duty maybe they can post details.
www.rhtltd.co.uk/productdetail.php?id=3&cid=10