First of all, are you hearing 60 cycle Mains hum? Follow this link and you will find a sound bite about half way down the page: http://en.wikipedia.org/wiki/Mains_hum
If this is what you are hearing, then the RF chokes and coiled wires suggested above will not help.
If it is 60 cycle hum then you can check the converter as follows. Disconnect the RV shore power. Disconnect the TV from the RV 120VAC outlet. Connect the TV to an extension cord that is connected to a 120VAC receptacle outside of the RV. Now run your tests.
If the problem goes away, then the most likely source is the power converter (60 cycle ripple on the DC lighting circuit). This is assuming that there is no power inverter involved.
If the problem remains, then I would suspect the cables between the TV and the stereo or a problem with the RCA jacks on the back of the stereo.