A float valve from a humidifier to your R/O unit should suffice. As a failsafe, you can use a float switch hooked up to a solenoid that cuts off the water supply to your R/O unit if the water level in your sump gets too high in conjunction with the float valve. If you are really worried, you can set up a couple of switches in case 1 should fail (if everything fails, you're just unlucky!
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