Jim -- Airstream certainly prewires their product for sound systems today. I don't know how far back this was done. Only enough responders with 1995, 1990 or earlier trailers would give an indication of how far back this goes. But I think it's almost certain it didn't happen in the 1970s. TomW's link is a more elegant solution than just putting a boom box in the front overhead like my PO did. I don't think there is anything sacred to prevent you from mounting speakers -- just put them where you wouldn't run into them.
I'd definitely think a 12 volt
automotive type radio-CD player would be the way to go. You could patch into wiring of any light -- those should all be 12 volts and you'd be fairly assured a PO didn't modify to 110V if they run off the battery without your Argo plugged in (I'd use my multimeter for sure). Do you still have the outside radio antenna on roadside front between the kitchen & pano windows?
You can dream up all sorts of options on an automotive radio-CD player, even including a subwoofer. I'd certainly consider something that is iPod compatible -- which about rules out a lot of Sony offerings. Probably time for a trip to Best Buy or other favorite vendor. One option is running wiring fore and aft under the metal water line guard at the roadside wall & floor junction between the couch and kitchen sink cabinet.
My first option would be to mount it inside the front overhead cabinet. Two different speaker options are shown below -- the top one would take some thinking but would work best with a midships location for the radio-CD player. The easiest 12V
source over the fridge might be a small line from the ceiling vent fan back to the room divider before turning toward the overhead above the fridge. The fan turned on might put some noise in the circuit. Time for a modern Fantastic Vent and maybe a new converter... All sorts of possibilities, eh? Next you'll wish for more than the one Group 24 battery...