I had a SL 273. I am making the assumption these are similar. The reason the speakers in the trailer don't play music or the like from the TV is because there is no audio connection between the two.
The electrical connection that is made is for lights, brakes and 12v power.
Got a 272 and I know inside speakers work win at last the DVD player that’s hooked to the tv. When we watch a movie it plays on speakers. I think on your DVD player it has zones for inside and out.