In the van is a very simple contained set-up, but it does rely on good water supply whilst out and about if you plan on topping up as you go.
We have always favoured a static set-up which can easily supply up to 3 vehicles from one system. Having said that some of our workers also have an 'on board' RO (Merlin) for fast topping up if they need it.
The static system is simpler to run and always gives you the ability to have a reserve, but van set-up does allow for a less space-consuming option.
In an ideal world I would have both, but realistically I know that the 'On board' set-up would virtually never get used, nice to have it there if you really needed it though.