Winnebago Minnie or Itasca Spirit of that era, you will have a 1/2 inch ball valve attached to elbow coming out of one side of the tank, wherever is your access to the tank, pump, and winterization intake. Metal lever about 3 inches long, parallel to valve to open, perpendicular to close.
Should not be under the RV, rather in a utility compartment if the tank is in the "basement" between floor and chassis, or inside the RV if the tank and pump are the RV (like under the bed or a dinette seat).