Been cleaning for 30 years and have always had on-board fresh water - 100 gallons. Unless you have a box truck, 100 gallons would be my limit. We have 2 setups - one in a Ford E-250, and the other in an Astro van (ETM). Both vans carry fresh water, both have inline filters and water softeners, and both have APO's.
I have an on-board water softener like Draper's, and an inline sediment filter, so wherever I fill up it is always soft water. I have a 30 GPM APO, so we enjoy continuous cleaning at every job.
When we arrive at a job, I check our water level to see if I will have enough water for cleaning without connecting to their water supply.
I also scope out their outside faucet location for closeness to the van, and will connect if it is close.
We go through some water as we run dual wands on just about every job.