DMan-
You might try Redlands Truck & RV, they are a sponsor here on iRV2.
If your rig has a Transfer Flow tank, built in Chico, CA (actually Orland which is next to Chico), then the sending unit is a standard 35ohm to 240ohm type available off Amazon (search Moeller brand fuel sender). Transfer Flow tanks will have a label on the forward end of the tank. Your particular replacment part will depend on orientation, top mount (in which case you have to drop the tank to replace sender) or side mount.
If you know about how much fuel you have on board, its a straight pro-ration (e.g. 1/2 tank is half way between 35 & 240), & you can check the pins on the sender w/an ohm meter to see if its working.
On my rig w/TransferFlow tank, my side-mount sender is visible thru an access hole in the frame on DS behind the FR tire. Turn the wheel to lock & I can crawl in to check ohms.
Hope this helps.
If sender is good, then you have a wiring or gauge problem.
Here is a good
thread discussing fuel gauge wiring of the era of your rigs.