I have to ask if your coach is built on a Freightliner chassis, because the fuel tank , and gauge package and related wiring are part of the chassis package.
I have the gauge testing info for my vintage of F/L chassis ; unfortunately they changed systems in 03, so it wouldn't be of use to you.
If your coach is on a F/L chassis then call the Freightliner Free help line with your serial number for info on testing your fuel gauge.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.