I use a double ended 3.5mm wire (headphone jacks) that takes sound from the iPod or Satellite radio dock and runs it into the media input on the stereo. Not ideal but works fine.
I don't think any iProduct will interface sound through a USB cable unless the receiver has isoftware built into it or a sync capability.
The other way would be to place it on a dock and then interface it to the camper stereo through some sort of output/ input.
2009 Jayco X23J
2011 Ford F150 XTR 4x4 Crew
Travel with wife, two boys, food and Canadian beer of course