There is a third tank for the galley. The handle to open/close the drain is sticking out of the frame on the street side in the area of the tires. My 2955 handle was just in front of the tires, my buddy's 2950 handle is in the same area. Yours will be in the same area. Look for the hole through the frame in case the handle has fallen off. The actual gate valve is close to the tank (some RVIA standard supposedly) and the tank sits over the axle area.
Trust me it is there. You are not the first to not know about the third tank.