First stop check that the vent line is not plugged as mentioned, the mud daubers like to build nests.
Fuel gauges invariably read level, not volume. The tank my be a 100 gallons in volume, but the chassis builder will reserve the top of the tank for expansion. It's likely that the fill neck goes in at the side or ends below the top of the tank. You know that your generator won't use the last ~¼ of the tank? Gauge. There is likely a recalibration procedure for the fuel sender, there is on mine. Because of my back ground the last 1/4 of the tank is for emergency, I never plan to get below 1/3. The top 2/3 is far longer than I want to drive in a day anyway. Who knows, the pick up may not feed the last 10-15 gallons unless there is sump in the tank. You should find out if your sender is a float or reads capacitance.