It's most likely assumed that when you winterize the unit that function is blown clear of water..I haven't seen your unit but..I would be willing to bet you could add a shutoff valve similar to what's used on water heater to shut it off. The trick is to do that and, at the same time, add a separate drain mechanism to clear the line and, yet allow you to use the rest of the water system internal.(If that's what you planning on..)
Find the line that feeds it from the inside and, put an adapter there for the drain..
It's possible there's one already there?
I' use my unit in the winter but never the internal water system.
Bottled water for me and camping ground comfort stations..Takes the worries out of possible freezes..
Maybe someone will pip in with the best answer.