GO up on roof...remove grey tank vent cap.... IS vent line roughly at roof line or has vent line dropped down into tank which then doesn't vent due to end of vent line being under water.
IF vent line is even with roof line then push garden hose with a pressure nozzle on it down the vent line as far as you can.
Turn on water supply and slowly pull hose back up vent line....have someone watch drain line/flow out of tank
Tanks slow to drain are due to venting issues especially when one has removed drain line and stuck hand/arm in tank.