If you understand how a system works you should be ok building your own Sean. Do you want RO in the van or will you be producing it separately in a shed or garage for example? 
Is there anybody local who could help you if you don't understand? 
 You need : Tank & tank outlet connecter reduced to 1/2'',
                    Pump, controller, leisure battery, 1/2''(12mm) hose to connect tank to pump then to hose reel. End stop for hose end, for connecting hose reel to pole. Reel and hose of course  

. And a decent pole. Plenty of threads on here for pole advice (or controversy  
 
 
)