First... Welcome to the forum.
The gauge shows 12vdc with everything hooked up outside the tank, but as soon as I drop the sending unit in the voltage at the tank drops to 1.25vdc.
Are you saying your taking a voltage reading at the tank sender unit?
Your Ohm reading at the sender sounds correct only if your tank is full.
Try Pulling your sender and testing it. Should read 0 empty and 30 full
The most common problem is a bad ground at the gauge cluster. I made this mistake myself.
The gauge must be grounded to the cluster.
Also check that the sender is grounded. On my 50 I ran a wire from the sender and it is grounded to the bottom of the Cab.
Good Luck
