It is common for the valve to be overtightened and for the plastic to plastic friction to make it very difficult to unscrew. Your photo is dark, but you can see the threads from the valve going into the threaded adapter. Unless you see glue on the threads where they enter the adapter, you should be able to unscrew the valve with channel locks or small pipe wrench. Worst case, the valve breaks and you can cut the pipe, add a new section with adapter, and replace the valve.
Larry, Eileen, and Finley
2004 Alpine 36FDDS
Third motor home, first Alpine, no need for another.