You will end up getting one in the end anyway - everyone does.
Yea that's pretty much it.
Go with an RO setup, since you are in a hardwater area, otherwise you spend to much money on DI resin.
Regardless of what system you will get, you need a static RO (in a shed or garage would be best) and a holding tank (preferably IBC 1000l). Transfer pump to whatever you will use during work. Ofcourse with a van system, you can have everything in the van, including the RO and DI vessel.
I'd go for a 300GPD RO, then in future you don't necessarily have to upgrade to a higher capacity RO. After the RO it goes through a small DI vessel to get rid of the last contaminants, so your storage tank will have pure water. With a van system generally speaking the water goes from the RO straight in the tank, then from tank to pump to DI vessel, still get pure water, but a little bit different then a static setup.
A regular semi will use about 20l apparantly (that's what I've been told) Still in the progress of switching over, so it's not from my own experience. So you will have to calculate what you need.
Also with trolley and backpack systems, you need to haul 25l barrels around. With a van system you don't.