You can listen to music with the ignition turned off, but only for 20 mins.
(The stereo has a timer to auto switch off to protect the battery)
So every 20 mins you have to press the power button on the stereo and you get another 20 mins.
The good news for RNS510 owners is that when it goes to sleep it doesn't go into deep sleep so restarting is quite quick - and if you are watching a DVD it restarts in the same place.
I don't find this too annoying, but I am not a big music player while camping - we only really use it on rainy days when we listen to the radio or watch a DVD.
There is talk of rewiring the stereo to the leisure batteries to get the positve feed from there, but I don't think that will fix it as it is the stereo that shuts down (probably when told to by CAN bus, not the power feed)
The way around all of this is to leave the ignition turned on, but then if you have daylight running lights then they will come on as well.
T