Cfing, you need to find out where the three video signals come into a switching box. This switching box is normally behind the dash. Once you find that box you can switch the three camera inputs around and see if the problem switches or not. After you switch the input cables around then if you can now see out that camera then the box is bad. First put some tape on those three cables to mark which plug they are plugged into so you don't mix them up.
I lost my drivers side camera and traced the video cable from where the cable comes from the camera through the front of the MH where the mirror was mounted into the generator compartment and found the one plug from the mirror camera to the video switching box above the generator and unplugged it and pluuged it up again and the camera started working again. Had a dirty or loose connection.
If it is the camera then these guys can fix it or get you a replacement. They are also good in helping to troubleshoot the problem.
RVCams Backup Cameras and Rear View Systems: Backup Camera Systems (All)