On my 2015 ACE 29.3 there is a switching module that determines which camera feed goes to the monitor. It has several wires run to it. It switches the cameras based on turn signal input. It is located under the access panel on the dash above the steering wheel. The three camera feeds are run to there. If yours is similar, you could swap the camera inputs to the switching module. If you get right camera video on the screen when plugged into left video input, then you know the right camera is working and it is probably the switching module. If no right camera video when using left camera input then there is an issue with the right side camera or the cable running to it.
On mine the right side camera cable is has no slack and is hard to reach. The left and rear have excess cable.
1998 Min Winnie, 2000 Winbago Journey, 2015 ACE 29.3
2016 Thor Miramar 34.2