If you've already got the means to carry a water tank, definitely worth it. Goto Tractor Supply if you've got one local, cheaper than the detailing company 'specialty' tanks. I've got $160 invested in a used pressure washer and 40 gallon tank.
I use about 7-10G per car, maybe 12-15 on a very dirty very large vehicle at 1.5GPM.
Jean Claude: 4 vehicles with 250 gallons? *** brotato, think about the dolphins! I did a 40 FT RV top to bottom almost twice with about 60 gallons.