I would totally agree with Gordon , It would be far easier to protect RO from the severe cold in the winter and Purifying water overnight into van you will need a ballcock valve fitted to ensure it does not overfill .
And if you have a full tank of water and the weather turns or you sustain an injury i would not want 400ltrs sitting in the back of my motor for 3-4 days .
A simple plastic storage chest under the kitchen window from Argos or B&Q to house RO can easily disconnected , You might have to spend about an extra £200 for water butt Sub Pump and hose but it might be worth it .
You could just hang once your system is fitted and just see how you go mate with it all fitted then take it out if you decide to do so .
Their is alot to mull over John just dont rush into anything without feeling you have thought things through .