I've never understood why anyone should want to carry all the weight (and value) of their RO system around in the van. I have a 300gpd RO-man in the shed, with a couple of 1000l IBC's. A sub pump fills my 400l van tank in about 5 minutes so all I have in the van is tank, pump, varistream, hosereel, battery and poles.
similar for me.
Once you've done your round a few times you will know how far and what you can do with your water anyway. A little bit of experience is all it needs.
If you keep running out then either your tank is not big enough, you haven't planned well enough or you are new to wfp and use much more water now than you will do in a few months.
A varistream or pole trigger will reduce usage considerably.
With a flow rate of 1 1/2 litres per minute working non stop for 8 hours solid (without breaks or travelling between jobs) would require 720 litres.
A 400 litre tank would allow you to work just under 4 1/2 hours of constant use with the above flow rate.
The above figures would double if 2 men were working together from 1 tank.
For domestic wc an onboard ro is probably unnecessary. There are more advantages for larger commercial wc.
Which type are you, do you work alone and how much money do you want to spend?