Lou is right in what he says in both of his posts; however, when I had my '07 Forest River Rockwood with probably the same Concertone setup as you, I was able to quickly wire up an easy work-around to make that old analog monitor work fine using the same kind of digital-to-analog converter box which Turbs is offering to you.
What I did was run a short piece of standard coax tv cable with F connectors on each end
from the Rockwood's combination tv antenna/satellite wall plate
to my converter box. Then I ran a standard red/white/yellow audio-video cable with RCA connectors on each end
from my converter box over to the Rockwood's audio-video input wall plate jacks
and plugged the red/white/yellow RCA leads into the matching colored Input 1 wall jacks.
I then powered up everything, pushed the antenna signal booster switch to on (green light illuminated) and tuned the Concertone control unit to the Input 1 Function setting. I could then watch today's digital over-the-air television broadcasts through my converter box to the existing analog monitor, and could use my converter box's remote control to change television channels, volume, etc.
I hope this reply post works out okay with the pics I've attempted to insert, as it's the first time I've attempted to post with pics.
Have a great time with your grandson, Jim!