Originally Posted by Mogulite
The filter under the galley sink is only for the filtered water near the galley sink and the ice maker. I think that's why there is a shutoff valve just before the filter.
So, it would appear that the shutoff valve is there to allow you to replace the filter without having to remove water pressure from the coach, as "60sumtin" surmised.
In that case, you will definitely want to remove the filter and replace it with a short piece of hose. Make sure the valve is open when you're done. That way the antifreeze can make it to the faucet and the ice-maker.
Either your coach owner's manual or the refrigerator manual (or perhaps both) should include steps on how to winterize the ice-maker with anti-freeze. If you read through de-winterizing posts you'll find that people frequently forget to do this, only to find in the spring the ice-maker and/or its supply piping have split open due to water left in them.