Do you have water in your garage? Any cold water supply line (like the one coming into your waterheater) is a great point to run your RO/DI from.
What I have going is my DI filtration set up in the garage. My top-off is running right off the output, switched through a DIY control unit that I'll get into in a minute. When the level in the sump drops, the unit kicks on until the sump fills up to the mark and I never hassle with evaporation, ever.
Here's my filtration unit sans RO. In Folsom, our tapwater is about 38 ppm TDS, so I only use a Sediment pre-filter, Carbon filter (for Chlorine), and DI resin in the last canister:
I make saltwater in 32 gallon Brute garbage cans and the rolling dollys make life easy. I fill the "Fresh" can with DI water and transfer that with a big powerhead to the "Salt" can where I add Instant Ocean to 1.025 and mix with a big powerhead and keep it heated until I need to do a change. I carry the now empty "fresh" can into the "fishroom" and drain about 30 gallons of old water from the tank into it. Then I wheel the "Salt" can around to the front porch and pump the new saltwater into the sump through the window through a long hose (my neighbors must think I'm nuts on water-change Sundays). Once everything's full I turn the tank's main pump on and pump the water back into the tank. Next I pump the dirty water back out through the window into the sewer clean-out near my front porch and away it goes.
Here's the thread on the ATO unit that I made, and it looks like there's enough interest in them that, after the WMC I will probably be making some more if I can get enough orders (been getting PM's up the wazoo lately).
http://reefcentral.com/forums/showthread.php?s=&threadid=682824
Since my water make-up is out in the garage, the top off unit version I'm using runs water into and out of a solenoid valve like your refridgerator's ice maker. When it needs water it turn on letting water flow, and when it's full, it shuts off. Works like a charm.
IF (big IF) I do in fact build these, or host a DIY so everyone can build their own, understand that I in now way assume responsibility for wet carpet and ruined hardwood. I'm taking a big enough chance with my own wife and I don't want to answer to yours

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