The info I have on gauge testing and wiring diagrams are for 99>02, so may not be of any help.
But if you call , Freightliner's , free help line , with you serial number, they should be able to provide the info you need for testing.
While talking with them , inquire about joining , Access Freightliner, for on-line access to parts and repair info specific to your chassis.
Just a thought, you may want to look across the top of your fuel tank to see if you have rodent damage to the wiring to the fuel tank sending unit.
Here is the info I have for gauge testing, but as you can see on the first page, it is for newer chassis.