Sounds like the power jack might be mis-wired, possibility of a bad battery too, or a blown circuit breaker. There is a circuit breaker on the tongue of the trailer that can get blown is someone hooks the battery up backwards. Does all of your other 12 volt stuff work on battery power only?
As far as the stereo, you can try to get the dealer to replace it. FWIW I use a TV with a separate DVD player.
Aaron
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