Toyota Repair Problems? Ask a Mechanic for Answers ASAP
Hello and welcome to just answer and allowing me to assist you. You are communicating with a real Toyota master tech, I don't know how handy you may be with checking automotive systems out so you'll have to let me know so I can walk you through this. The easiest way to illuminate one or the other if it's the gauge or sending unit is to lift the rear passenger seat then the cover plate to get to the connectors to the sending unit. with the key in on position disconnect the connector. If the gauge goes down the problem is in the sending unit in the fuel tank. I will send a pic to help you out.
A pic will be very helpful
here is the pic of the sending unit' it could be either stuck in the upper most position or shorted out. I also noted you stated you just purchased this just about a month ago? why don't you take it back? When you lift the rear passenger lower seat out you will see a metal cover plate that you have to remove to gain access to the connector.
Going to bed right now but will be available tomorrow to continue if you need to do so.