[SS Tim]

Pictures always help, but I found yours on another post. The stock tank would be behind the seat in the cab. A factory installed optional tank would be installed on the drivers side under the bed with a filler going up through the bed and out the side. It will also have a selector switch on the far left dash near the door. Saddle/Aux. tanks were offered both as a GM accessory to be dealer installed or aftermarket and there are many variations. Some have doors cut in the bedside to access the filler and others had the filler in the wheel well that often as not has a can over the top to keep, er... stuff out of the tank during fueling. Most mounted under the bed, some were inside the bed forward of the wheel wells. Generally all the aftermarket installations used a manual selector valve on the floor. It was cheap, functional and universal. The method of operation was to fill the saddle tanks and drive them dry then switch to the factory tank last as generally it was the only one with a gauge.
These were often installed on campers to get past the slingshot range due to sometimes single digit gas mileage in an generally overloaded camper truck.

Looks like your system might be a combination of parts. LL is likely left tank. RR right and MM cab tank. Best thing to do is chase down and draw out your plumbing from the valve. Don't move the selector around until you know the tanks and lines are clean. Many times these tanks fall into disuse and nothing remaining inside should ever go through a carb.