Just a thought....Its early and my thought process may be flawed but you may be able to unplug a known functional side camera line on the camera controller and plug into the rear camera connection on the camera controller. This would remove the rear camera from the system. If the side camera now works, and shows on the monitor something in the rear camera/coax line is amiss. If the side camera does not work, its could be the camera controller. This at least would narrow down the possibilities.
Stu & Nina Stoner
Central PA (Lewisberry)
2017 Jayco Alante 26X
2018 Thor Four Winds 24F (traded)
2016 Forest River 22RR (traded)
2008 Honda Gold Wing Level 3