Yeah, I'm in Ca. - Central San Joaquin valley. You?

I checked out some of the RO systems for hydro on Ebay. Pretty much exactly as I described if you did it yourself they just use dual membranes to get the 100-200 gallons per day. And the price is right so you wouldn't save that much by building yourself.
Alternatively look online for the "Merlin Reverse Osmosis System", it does over 700 gallons per day and if you shop right you can get it for around $400. That would fill your res pretty quick.

If you are just a little handy or have a friend who is, then installing one of these is not hard, and if you get stuck then you could always call a pro.

Best of luck, let me know what you decide and how it goes with the install and daily use of the system.