So you have 2 grey tanks????
One for shower/bathroom sink and one for kitchen sink?
IF so.......
Your shower/bathroom sink grey tank VENT to roof is blocked/obstructed.
The 'valves' under sink (bathroom should have one also) are to prevent sink 'P' traps from getting sucked dry when draining grey waste tanks.
Each waste holding tank has a vent to roof to allow air in/out of tank when filling/draining.
IF vent is plugged tank will struggle to drain due to lack of venting.
Need to clear vent line.
Open tank drain....go up on roof & remove roof capon vent line.....stick garden hose down vent line........blast away with garden hose
