I did a lot of research about replacing the head end unit in my previous RV. I finally figured out how they handled the 3 camera inputs when the stereo only had one. They used something like this:
RV Cams CVS300-01 Three-Camera Switcher With Touch-Screen Control
It basically wires in to the turn signals and the reverse signal and the 3 cameras. Then has one output signal and camera output for the stereo. When you put the gear in reverse or turned on one of the turn signal, the stereo would switch to the correct input.
Something like this would open up what type of stereo you could use, since you'd only need one camera input.