gray water tank

I have a Montana Mountaineer which is very similar to the Cougar.
If you have a galley tank then the drain valve is not with the black and gray tank valves. My galley valve is under the camper near the sewer hose connection.

When you opened the grey water drain valve did water flow or just trickle out?

Are you sure you do not have another grey drain valve.....back behind wheels/under slide??

If water flowed then sink drain line could be plugged up.
If water only trickled out then tank outlet drain hole could be plugged. Sometimes the cut-out disc (from installing drain/vent lines) falls into tank and is not removed......floats around until it finally gets sucked down to outlet drain opening and blocks the drain.

Get a clear adapter that goes on end of drain line and then sewer drain hose connects to adapter.
With the clear adapter you can see how well everything is flowing when draining.

Tanks & drain lines are just like plumbing in S&B house......ABS. So you can snake the drain lines, use drain cleaners etc.
BUT if you use any drain cleaners drain & flush lines/tank immediately after use. Drain cleaners can be harmful to the drain valve seals.

yes, not me.