Need to maintain 5v to load during supply switch over
You do not need a relay on the ground side.

You may get away with a single relay and two SCHOTTKY 60V 5A DO201AD diodes acting as bypass while the relay moves as shown below. It will depend if the camera can handle the voltage dropping to about 4.7V briefly. They are cheap though so a quick test is easy enough. NOTE: You may not even need the relay with this circuit.

However, your issue may be a sequencing one. If the battery pack shuts off the outputs before the USB adaptor completely comes up it may still be an issue. If so you will need to add a large capacitor in after the diodes.







simulate this circuit &ndash; Schematic created using CircuitLab