Personally, would do a hydor smart level control with a pump or a solenoid valve. Holds on with a strong magnet, alarm for low, alarm for high, and no moving parts. Just gotta make sure you rig it so it get's from low to high within 10 minutes, or it sets an alarm and shuts off.
if you are filling it through you ro/di unit a switch will not stop the water, i use this one my customers top off water container
http://www.bulkreefsupply.com/auto-shut-off-kit-for-reverse-osmosis-systems.html
Nice. I'm going to add another float to do this eventually. Like this.Im going from RODI to Ato reservoir but i have a controller from autotopoff with latching relay. One float switch is set on the low mark and another float swirch on the high mark. This is all connected to the controller and a Solenoid. When water level hits low mark it opens the solenoid letting water frim Rodi to fill reservoir when it hits the top about 5-6 gallons later it triggers the upper float which shuts off the solenoid and process repeats itself so no TDS creep issues and burning Resins.