Need some guidance on fuel tanks - 85 Eldorado President

First thought is the aux tank is empty even thought the gauge says otherwise.

Next, dual tanks typically use a solenoid to reroute fuel between tanks. It may be dead. A replacement (even generic) should be easy to find and plumb & wire in. Follow fuel line from one of the tanks till you find the solenoid.

Also, the line from the aux tank could be plugged or cracked/split.

Best of Luck!