dougrainer has it correct; it's likely that the previous owner changed out the fuel pump assembly with a simpler one without the generator pickup. Go to rockauto.com, and search for fuel pumps for your chassis model; they likely will list one for your chassis/fuel tank combo that DOES have the generator pickup. The "port" next to the filler neck on the tank is likely a vent. Have you tried drawing fuel from the line on the fuel pump assembly (that currently goes to vent at the top of the filler hose)?
BTW, the only way the return line would work as a generator feed is if it extends 3/4 of the way down into the tank.