Bazzy, not sure where you are up to yet but have you not considered fitting your RO and pre filters to a a holding tank in your shed or garage?
I had the same set up as you appear to be setting up in my van where the whole set up was in the van. I had a couple of jobs where my 400litre tank was used up halfway through the day. I had to drive van home and attach to tap waiting 4 hours for van to fill up before returning (the next day!)
I decided to take out and set up the pre filters and RO in my shed attached to 2 linked water butts. These are constantly full or refilling when I am at work.
I just nip back and using a submersible pump in the butts, I refill my tank in 5 minutes from drive in to drive out. Before I drive away, I turn the tap on again so water starts its purification process agin into my water butts.