With it being intermittent and you have no other issues along with it - It is most likely the Camera.
There should be a code generated form the BCM, you can do some Wiring Checks, but more importantly what are you doing to get it to work. --Key Cycle?
The code generated may clear on its own so you would have to scan with the fault present.
The wiring checks for voltage and resistance may not show any discrepancies unless you do a shake test of the harness while doing it.
Optional way to check the wiring, with it faulted, shake the wire harness where it exits the tailgate and down the harness a few feet with a helper watching the display and note if the display of the camera flickers on/off.
Then open the tailgate access panel and shake the harness near the camera connector.
Then move on with a Voltage Check at the camera connector.
Ref: This detailed thread on a similar issue with the camera inoperative.
Lots of wiring details and check for power, the only topic it does not cover is the video feed to the APIM.
