Not necessarily... as someone who has done it twice in the same car. The engine ran fine after a drain down, right up to the point that someone drove straight into the front of it.i know petrol in a diesel engine menas new engine ...
Cylinder head gaskets gone, its steam.
True enough, but it depends on the mix, since there will always be a proportion of petrol still in the tank to dilute the diesel. Anyway the engine will still run for several miles before seizing. You have a chance - though expensive! (I should point out that I have no first-hand experience of this: though my father once had a bag of sugar emptied into his tank by some little scrote or whoever. Cost him a new engine...)
Its a van so i'm presuming its a diesel. Fuel pump timing problems will produce white smoke, but I have never seen that problem produce anything like that. Has to be head gasket, so one of two things, oil getting in, it would be notably blueish and absolutely stink, so i still reckon its water.
In the old days truck drivers used to add petrol to their diesel tanks to prevent waxing in cold weather.