Ford F 450, 1997, gas gauge works, but switch won't go the second rear tank?

I replaced the switch same thing, only reads front tank and rear tank stays full.
I had the solenoid replaced on the frame 15 months ago, could that be bad again, that is over $400?

Added (1). I have two gas tanks, front and rear. Switch won't switch to rear tank?
Reads front tank only, even when switched to rear tank.
I replaced the switch, cost $62.

Added (2). The rear tank is full, Edith. It is full because I can't access the fuel.

It is the switch/sensors inside the gas tank. Or your rear tank is full.

No