If your side cams still work that narrows it down to the rear cam, the cable or the input on the Connexx switcher.
Should it be the Connexx switcher, here is the Entegra part info:
0256097 CONTROL,3 CAMERA AUTO SWITCH WITH MINI FIT CAMERA CONN CVS375-02
You can probably get it cheaper elsewhere, try rvcams.com.
Tim at rvcams may also be able to help you troubleshoot and identify the failed part.
Here's another photo of the Connexx switcher: