When I used to set-up systems in homes/offices, in most cases I was not able to have an extra container for freshwater storage for evap top-off. I used a Liquid Level Controller (LLC) from Spectra Pure. This allows you to plumb your RO unit right under your tank/stand and turns on/off your RO which feeds directly into your sump.
I personally dislike storing water as it takes up space, it's heavy, and it absorbs impurities from the air (especially RO/DI water as it's hungry to re-ionize).
This LLC system is electronic and by default, it's shut off when there is no power. It works by a sensor (same vacuum sensor used in your laundry washing machine) installed in your sump at the desired water level. Once the water level drops below the sensor, it opens the valve installed in front of the RO unit allowing the water to feed through and refill sump.
The down side of this system is that you'll have to plumb (2) 1/4" polypro lines, to and from your RO unit ((1) for supply, (1) for waste water). This is very easy to do, it just requires some planning/foresight. I used the 1/4" self piercing saddle values for HD/Lowes, and works on copper or soft pipe...
I found this system to work flawlessly, and I installed probably around 2-3 dozen of them. It also has a mechanical float valve for safety, JIC.
I find mechanical float values will fail - it's just a matter of time until they do... Like stick open... Which will flood your sump and cause it to overflow at the least opportune time (while you sleep or while you are on vacation) Or, stick closed and burn up your pump(s)/heater, etc.
Here's the link and I hope this helps: http://www.spectrapure.com/St_alc_p1.htm