J.R., my first question is how old, exactly, is your Pace Arrow? In the 80's and 90's, Fleetwood typically designated the "main" fuel tank as the one that came with the chassis. It was usually the one centered between the frame rails. The auxiliary tank is most likely situated to one side or the other, depending on the floor plan. The main tank is usually the larger of the two containers as well. But your problem may not be in the fuel containers. The older Pace Arrows had an electric switchover valve that enabled you to choose which tank to operate from; hence your dash switch. If the solenoid portion of the valve becomes faulty, or the passage way blocked, you'll be unable to switch between tanks. Have a technician test this device. It will be located on one of the side frame rails and have either three or six fuel hoses attached to it. Many RVers have added an auxiliary12-volt fuel pump to help get fuel from the main tank up and to the carburetor. Also, some fuel tanks had an internal pump that can also fail. I've always favored the outboard type that can be easily serviced. But, you do need to have fuel getting through that changeover valve before it reaches the engine.
Hope this helps! If not, send me the actual year and model of your Pace Arrow.