I have been researching the replacement of my cheap Jensen unit with something better. Camera access is important to me and everything I have found has had a single (or at most two) Video inputs for cameras. Well I have left, right and rear. I found this item on Amazon that will control (via trigger wires) up to 4 cameras and send them to a single video output.
I don't know if this will be plug and play with all cameras but the camera connections seem compatible with my Forest River Georgetown.
This will be a winter project for me so I haven't done it yet.