Grey Water Tank Won't Drain

[dickcarl]

Working the kinks out of a new-to-us '88 Itasca.  Somewhat strangely, it has a grey water tank for the shower, and a grey water tank for the kitchen sink.  Water from the bath appears to drain into the toilet black water tank.

The shower drain 1.5" drains just fine when you open the gate valve.  So does the black water.  But the "sink" tank hasn't been draining, and I realized this when Beloved Wife pointed to 2" of grey icky water in the sink.  Tried a plunger, snake, etc.  Finally went out with the snake and found out it only went up about 3" -- right to where the gate valve was.

Is my only option to physically remove the 6-8 screws holding the gate valve together?  The handle moves in and out nicely, so I'm assuming it's come loose.  But it's VERY had to get to, wondered if there were any tricks...