so, assuming your home water is essentially "free" (tap), and you might spent 20 dollars on buying water closer (as stated, very iffy to find anyone along the way who will let you fill for free), do you really think it'd cost 20 extra dollars in gas to haul the water over the mountains, and through the woods? Not to mention the time to stop and look?
and, as you imply, it may take a little research, to find anywhere to camp, but it would seem said research is "on you". Though you may get lucky and someone will know specifically.
To that: I'd imagine, if you get off "reservation land", and are not on private property, a large part of Nevada is "BLM" administered, and is pretty open to camping. I'd check their website for specific rules though.