Check what your van can carry, you need to take into account your tank an fixings say 50kg, your water 450 litres + 450kils, your self and your matey if you have you have on and all your tools.
When fixing dont just pop a bolt a bolt through the floor pan, it needs to be fixed to a secure point, just take a look at the mountings for a seat belt, they a reinforced and they only have to stop a body, not 450 kgs of water.
THINK BEFORE YOU DRILL BEFORE YOU FIX.
steve@freshwatersystems.co.uk