I prefer gravity for my ATO .
I have my RODI hooked to an solenoid vlave that the APEX runs.... once a day the apex turns on the valve and the rdoi starts making water. That water goes to a 2.5 gallon container above my one sump and then once its full it gets shut off from a float valve. the wate rthen flows to the other top up container for my other tank... again with a float valve to close the water flow when full.
The sums in each system have another float valve that opens and closes as the water level decreases and fills back up, draiing the 2.5 gallon containers. Next day the RODI turns back on again and refills th2 containers again.
I haven't looked at topping off water in 6 months.