I understand that the USB port of the Arduino mega is link to the TX0/RX0.

I am trying to connect a serial camera for testing but it needs a USB-serial converter.

Is it possible to use this configuration as a USB-serial converter i.e. I connect the camera TX/RX to Arduino RX0/TX0 and USB to PC?

I am a bit confused as to what exactly you want. The arduino mega has four serial ports. If your camera has a TTL serial output connect it to one of the other ports and run a sketch that transfers the bytes as they arrive.

I have problem sync and getting a response from the camera thru the sketch.

I am trying to use the camera demo program that needs the camera to be connected to the PC. In this case, I will need a serial to USB converter.

I read the words but sadly they do not make any sense.