The ground connection that completes the circuit for the fuel level sensor is from the sensor flange to the fuel tank and then from the fuel tank to the air frame. If the removal and reinstallation of the fuel tank resulted in a slight electrical resistance between the tank and the airframe it would have an influence on the accuracy of the fuel level indication.
Use an ohm meter to check the resistance between the sensor flange and the air frame. If there is anything except for zero ohms that is your problem.
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Van's Aircraft Engineering Prototype Shop Manager
RV-6A (aka "Junkyard Special ")