I was under the impression that most Airstreams have a winterization kit already installed (though in mine and many others, the valve was installed backwards
If the 25' FC is like the 27' Globetrotter, the water pump is under the closet floor with a bunch of other stuff. Look for a clear tubing (probably just shoved in there)—one end will be unattached (and will go in your antifreeze jug) and the other will lead to the valve.
If you don't already have a winterizing kit installed either the closet floor has to come up or you'll need tiny hands (and perhaps a pex cutter, clamps, and crimper).
My valve was installed backwards and I had to remove, reverse, and reinstall it.
While you are at it, you might want to install a pressure tank. Makes the whole unit much quieter if you can isolate the tank and hoses and gives you enough water to do quick things in the sink even after you've turned the water off.