Would it be possible to find the hot wire that powers the radio/CD, and wire it to where it's not on the ignition key circuit? Check on your 12V fuse panel as there may be a few sections open. I found several open ones to use on mine and connected a musical horn.
My radio works all the time key on or off.
Lawrosa has a great idea too. We use the AUX input with the TV, Wii, DVD player,Pandora on the smart phone, and our Bose radio to route the sound from these through the roof speakers. While driving we can have the TV on and put the sound to the back speakers so the kids can hear and we don't have to yell over the TV.
Good luck.
Coachmen Mirada 310DS named "The Corporate Headquarters."
When camping I just tell clients that I am working at "Corporate Headquarters"