That's what I did. The G armin 770 (I think) has room for a camera. That one is mounted above the one built into the coach, and can be left on all the time. The other shows the sides and back up when in reverse.
I disconnected all three cameras from the box and took the box out, then added a 9 inch screen with multiple camera inputs to the dash that allowed me to display all 3 camera views all the time. Now the rear view displays across the top half of the screen, and the left and right cameras split the bottom half. I love it, and the in-dash radio screen no longer is involved.
Screen, $70 from Amazon, is podofo 9" TFT LCD Split Screen Quad Monitor.
I did the same thing. Just finished an LA to Atlanta drive and got very comfortable with the configuration. I did realize that I didnít really need the side camera views as they took up space on the monitor and didnít give me any more information than my mirrors. So I put the rear camera view exclusively on the monitor. I could see any vehicles from that view who were speeding up behind me and when my trailer was clear when changing lanes. I did run into visibility issues in the monitor when the sun was behind me or at night when headlights were shining. Turning the brightness on the monitor all the way to 0 helped a lot.