The service manual with my '77 says to fill the tank all the way up, press the rocker switch to indicate level and then use a small, thin screwdrive through the hole provided in the control panel to move the needle to full. I'm not sure if this procedure works with yours but it seems feasible. I would want to know if the thing was working toward full more than empty. Also be aware that battery voltage/strength affects the settings. I have flushed my grey and black water tanks within the last several weeks and will be adding probe cleaner the middle of next month in preparation for a June 28-30 outing.
'01 2500hd ext. cab, 8.1 litre gas, 5 sp. Allison auto
3.73 rear end
Mag-Hytec rear diff cover
Amsoil Dual by-pass oil filtration system
Amsoil synthetics all around
265 watt AM Solar, Inc. system