Most important point: What's the van's payload? Have you considered what size tank to put in? Are you going to fit the RO in the van as well?
Ian's is a good point, also make sure the tank is baffled. water is 1kb per litre, tanks can weight over 30kg empty, hoses di ro,, poles etc etc all weigh a bit aswell, say 1000kg pay load IMO a 500l tank, weigh bridge is expensive if you overload and also very dangerous and invalidates your insurance.
I would have the tank installed by a professional installer:
www.purefreedom.co.uk do this about £300 htis service could save your life if in a crash, also looks the part, I have a few pics of there work if you want to have a look let me know and I will post them on here for you.
IMO I would have hand held Tds meter and not an inline (these are prone to give incorrect readings)
have you ordered your system yet ?
if you want to have a chat with me about it give me a call on 07788 732 789 or 01234 870 225, you will not be the first that has called and asked about this, I dont sell them so will give you an honest answer without the sales patter.
best wishes
Ian Wicks
www.windowwashers.co.uk