Gray tank plugged, limited options

There is no rule that states the 1.5 inch vent pipe has to go straight up from the tank to the roof. Even if there is a Y in the wall, you can probably snake a tube down to pump it out. IF you have 2 tanks and 2 roof vents, then no Y. IF you have 3 tanks and 3 vents no Y. BUT, they may have a slight ABS elbow going up, but that would still allow a snake hose to go into the tank. What I do when I suspect or verify a Valve has broken loose from the handle rod. I drill a 1/4 inch hole thru the center of the valve 180 degrees opposite of the original handle and then run a 1/4 steel rod into that hole and push the knife valve open. The 1/4 inch steel rod minimizes any leakage. You leave it partially in the drilled hole to seal, until the tank is empty. Then you have a empty tank when you replace the valve. Not sure if you stated, do you have a mechanical manual HANDLE or you have the Cable driven Dump Valve? If cable driven you can usually access the pull rod where the cable attaches at the valve and pull it open. Doug