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.
Is it time for your medication or mine?
2007 DODGE 3500 QC SRW 5.9L CTD In-Bed 'quiet gen'
2007 HitchHiker II 32.5 UKTG 2000W Xantex Inverter
US NAVY------USS Decatur DDG31