their are ro/di systems that have a 3-5 gallon preshure tank that stays filled under the sink with a separate spigot that you mount above the sink for drinking water, you could use a system like that with a T and a valve off of the line to the tank and use that t to get your tank water. when you need water for the tank hook up to the t and open the valve and close the valve on the preshure tank
when i bought mine years ago their wer only a few companys making the membrains ge/osmotech and filmtech and most ro systems can use bolth, not shure if this is still the case though since that was several years ago