When Boondocking Water can vary from free to expensive, and the desert Southwest is one of the places where it's often expensive. Some gas stations and truck stops will have water available but in the desert areas they will often charge you for it and five dollars to fill up is not uncommon. If you stay at the long term visitor areas in Arizona and Southern California water will usually be available as part of the entrance fee.
Since a lot of folks don't like to move there RV once It's set up, bringing water to the RV is more common than you might think and can vary from complicated, like tanks and a pump in the back of a truck, to the very simple, like 1 gallon or 5 gallon water containers that you pour into the RV. It's actually a fairly common sight in the desert areas to see a pickup truck with 15 or 20 one gallon jugs of water in the back. The blue 5 gallon water jugs that you get at Walmart are also very popular. And these water jugs are often filled up at kiosks that sell water for $.25 a gallon.
Googling up locations for water is a good way to find where water is available for RVs in the desert areas and there's a lot of shared information available.
Now of course when you get out of the desert areas waters becomes almost a nonissue, with it being available at a lot of gas stations, truck stops, rest areas, and parks for free.
Having an RV that is designed for Boondocking or as a full-time RV is also very helpful because they will normally have large water tanks. For instance my Arctic Fox 27 – 5L has a 73 gallon water tank plus a 10 gallon hot water tank, which gives me 83 gallons of water to use. So by using just basic water conservation measures I can easily stay boondocked for a month.
The taste of water will vary considerably, especially in the desert areas. Some folks buy bottled water for drinking and use the water in the RV tank for washing dishes and taking showers.
It's also very common to have a water filter under the kitchen sink and only drink filtered water. I bought an Everpure water filter a while back but haven't got around to installing it yet, I guess I'm just not very picky about the water I drink.
If you have any questions about Boondocking and water, please feel free to ask.