[rohrmann]

Your black tank is on the same side of your rig as the toilet, so that would be the door side. The black and grey tanks are aligned front to rear, and the drain valves on both are at the rear of the tanks. You can get to the valves by removing some of the black coroplast from the rear and some of the side. The black valve has the cable assembly leaving the valve toward the convenience center on the opposite door side.