Many bikes have had this problem and here's the fix...

Check the float in the tank. They are known for hanging up. You will have to bend the arm in a way that repositions the float away from the tank and the hoses around the unit (not up or down) but sideways. Bending it up or down will change the readings on the gauge and make it inacurate. You have to pull the panel on top of the tank to do this. Just remember to replace the screws and gasket when you do this, they are self sealing and cannot be reused safely. If not that , then here's a post that will give you the parameters for testing the sending unit..
https://www.hdforums.com/m_66687/tm.htm

If you also experience a false reading on the guage that puts the gas in the tank about half full when the reserve light goes on, the solution is to bend the float rod about 20* downward to correct this condition.


Here's a link to a tech tip on the proceedure for the float rod (page 2)... HERE