Dual gas tank problem on P/U

My 88' F-250 I just bought has two gas tanks.  I've been using
the front one, but when I hit the toggle switch to use the
rear one, the gauge falls to empty, even though the tank is
full.  I assume that's an electrical problem.
Then, after a half mile or so, it will begin to hesitate,
as if it's not getting quite enough gas to run.
I then quickly switch it back to the front tank so I
don't mess up the injectors.
Does this sound like bad gas maybe in the rear tank?
Or maybe a clogged line?
I don't know how old the gas is in this rear tank...the guy
lied and said it all worked fine when he sold it, the gas
may be very old if he wasn't able to use the rear tank.
If I need to drain the tank, how do I do that?

Thanks,
Rich