I'm desiring the same thing - I had a mirror in the old Four Winds - and a Fresnel lens in the rear window. It was a good setup.
In the new Sunseeker, I have the backup camera monitor in place of the mirror, as does the OP. I cannot see back into the coach like I used to do - "What's that loud noise back there - did the refer just dump its contents?" I'm considering moving the backup monitor up to the space where the cab light is located now on our Chevy chassis, and installing a standard rear view mirror. This would help a couple things - I would be able to see back into the coach, and the mirror size is smaller than the backup monitor, so less of the view is obstructed. It's a project for another day.
Glad to know I'm not the only one...
Formerly: 2000 Four Winds 26Q/Ford, '96 Kit Sportsmaster 22' 5th wheel, '91 Sunlite truck camper, Wildernest flip-top canopy