First and foremost, you need to find out what TDS reading is coming from your water supply. Then you know which system you need to purify your water. Anything above 50ppm and I'd recommend a RO system, below, just a twin DI set up will be fine.
Without using ladders, WFP is the only way, unless you want to set up scaffolding up at every job!!!
No matter if you need DI only or RO system (again depending on TDS reading), a 500L tank in your van will/should last you all day at the correct pressure settings, once you are upto speed.