This is a pretty simple and easy DIY auto top off that can be modified to fit many situations.
I have my RO/DI system in the basement. Instead of buying a couple of hundred dollar pump to fill the sumps I am running it directly off the RO unit. Note - some people do not reccomend this as the RO membrane may pass some bad water when first started up.
I put a tee on the good water discharge from my RO/DI. One end I ran to my storage bucket. The other goes up through the floor to the back of the tank stand.
The tubing runs through a ball valve, the solenoid valve, from McMaster Carr p/n 7877K53, and to a float valve in the sump. The solenoid is powered by a digital timer. Then both the timer and float are are on water will fill the sump. This gives the failsafe backup of having two devices call for water in case one fails.
I have it set up so that twice a day for 30 minutes the solenoid is energized. For me that is plenty to keep the sump filled.