Hi Chris,
You do not need to 'polish' the water in your vehicle.
We have ro units at our base which process the water we then have the product go through a fianl polishing canister (ro-man type) to remove the final 002ppm tds and it is then stored in our 1000litre tank.
We have no polishing at all in our vehicle. This keeps the vehicle side of things a lot simpler.
Alex