If your fuel tank is actually empty but the gauge shows as full and when you fill up your tank and it shows as empty, this means you picked up the wrong fuel sending unit.
Renix and Chrysler fuel sending units are backwards or reversed to each other. So if you have a 1991 or newer model and having this problem, chances are you installed a renix fuel sending unit and vice versa.
Recommend always buying Bosch or Carter fuel pumps. Airtex are cheap but you get what you pay for.