You do not have to have a tank in your RO system to have an auto shut off valve (ASOV)
Most systems come with this valve so that the unit shuts itself off and does not make any more ro water nor any more water to the drain.
Here is a link, for a picture, there are a lot of pics on the web.
http://www.goodwaterwarehouse.com/cat/6_roasv.shtml