Unfortunately, I can't run my RO
line to my top off reservoir or sump becasue of their relative locations. (I don't think my wife would like seeing a 1/4" white line running down the hallway to through the living room to the tank.) I do have a plastic bin that holds about 3 gallons sitting in the stand beside my sump. I add my RODI
water to this. I then use a Tom's Aqualifter pump controlled by a DIY auto top off set up to transfer the water to the sump as needed. If I had to do it over, I would havce gotten the auto top off from autotopoff.com. It looks a lot nicer and is not much more than the DIY kit I got, plus you don't have to sacrifice an extension cord. Get the one wit two float switches. The lower one turns the pump on when the water level drops and the upper one is a safety to turn the pump off in case the lower float switch fails. I think the DIY kit was about $40 and the autotopff.com switch was $50. The aqualifter pump you can get for about $16.
I know you can use a mechanical float switch to run your RODI
water directly to your sump, but I'd be afraid of the float gettgin stuck and creating a flood. I seem to think someone makes a controller that gives you another level of security if you go this route, but I can't twll you where I've seen it.