Diagnostics.
The fuel level transmitter (float sending unit) operates on a SPECIFIC ohm range. The lower the actual fuel level the higher the resistance. Pure digital fuel gauges are incredibly sensitive and need a tool with LED's to diagnose. A needle mechanical gauge is easier. Easier does not mean easy.
It's not the dashboard needle causing the problem. It's inside the tank or inside a harness connector located one to three feet away from the float, sending unit and possibly the gasoline fuel pump (a combination unit). The combination unit is incredibly expensive so banish the thought of just replacing the whole smear to fix the problem.
Crawl under the rig with a flashlight. Try and find one or more wire harnesses that run along the frame then disappear on top of the tank. With a pad and pencil draw a scribble diagram showing the insulation color of all wires, but you need to also check each wire for a thin stripe of color. Brown wire green stripe for instance, Wear goggles as dirt can fall into your eyes. It takes me a half hour to pry apart a still connected wire bundle and twist each wire to get all the colors down accurately. You are undoubtedly quicker. Don't worry about wire orientation in the plug that comes later.
NEXT STEP: Go to the dealer. Go online. Search. You need to discover which color wire is the one going to the fuel level "sending unit (float)" in the tank. The dealer may be reluctant to furnish this information. Go online and search for wiring diagrams and circuits for your exact rig. Yes this is a royal PITA but you cannot do the job without finding that particular wire (color code). Next, find the color of the ground wire. It is usually black.
You found it? OK. You're going to need a 12 volt test light, some rough sandpaper to scratch and buff out a ground connection. First, buff the point of the test light to needle sharpness with the sandpaper. It has to be like a hypodermic needle to pierce hard plastic wire insulation.
Pliers. To hold the one suspect wire as you pierce wire insulation. Fingers don't work so cool for this.
Connect the test light ground clip to the shined up part of the frame you scrubbed with the sandpaper. You need to Shanghai a friend or spouse for a few minutes to operate the ignition key and watch the fuel gauge as you work underneath at the tank.
Find that fuel float sending unit wire. Ready? Pierce the insulation with the test light probe tip. Yell "Turn the ignition switch to On and watch the fuel gauge!" Touch the tip of the connected to frame test light to the hole you made in the insulation. Within 2 seconds, the fuel gauge should rapidly climb toward full. Don't leave the test probe connected for longer than 2 seconds to eliminate possibility of hurting the gauge. The bulb in the test light acts as a safety resistor. Do not try this with a plain jumper wire to ground. Do not expect the test light to glow during the test. The light is nothing more than a resistor/probe.
Gauge still dead? Time to empty the gas tank and drop it. A big empty tank is a big job and a tiny full tank is an impossible job. The tank must be empty or near empty.
I am stopping here because it would be pointless to continue if you won't or can't do any of this...
