I started out with whole system in the van.
After a couple of commercial jobs where I had to drive back the next day to finish off because I ran out of water and the on board set up took 4 hours to fill 400 litres...I took out the RO and pre filters, screwed them to my shed wall and sitting underneath are 2 x 220 litre water butts linked together.
Inside one of the the butts is a submersible pumpth 10 metres of 4cm hose attached.
I now have a constant supply of low TDS (average 5-10tds) water sitting in the butts while I am out workingback van up to shed.
When a fill up is necessary, I can now do it in minutes either during the day or at the beginning of a new working day.
I back van up to shed...
...unscrew cap off van tank
put in hose from submersible...
plug in submersible to outdoor socket...
van is full in 2-3 minutes...

After the van tank is a pump and a vessel of DI resin before the hose reel to bring water down to 000TDS.
Another major advantage to a shed set up is becoming aware when you are dismantling your van set up as to how amazingly simple a purification set up is.
You will probably curse a bit if you have paid a supplier a fortune to have it installed but you will know in the future that DIY is so simple and so much cheaper and will do EXACTLY the same job.