Well I wanted to confirm to the group this can be done. There is some mention of it above, but I wanted to describe my personal results.
1. We have a 2012 27FB International Serenity
2. The Sony headunit had subwoofer outs, and front and rear "outputs" which were non-used RCA connections.
3. I purchased the Micca transmitting unit because of its 150' range claims and it has aptx. (more on that later)
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
4. We also purchased a bluetooth aptx compatible speaker, one that you can plug into the outlet on the outside of the trailer, but also has an 8 hour battery life if you wanted that too.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1 It currently took a price drop on amazon and is a pretty good deal IMO.
5. After connecting the Micca unit to the Sony, and then easily pairing it to the bluetooth speaker it works GREAT! Now we can listen to anything the indoor sony is playing both indoors and outdoors. I would highly recommend this.
Now to my final point, I would highly suggest you only do something like this if you get the aptx low latency transmitter and bluetooth speaker, (or very little delay between wired speaker output and bluetooth speaker output) I also paired an older bluetooth "jambox" we had as a test and it did not have the aptx feature like the speaker mentioned above. The delay between when you hear the sound inside vs. on the bluetooth speaker is bad enough it will drive you nuts. There is minimal delay when using aptx bluetooth, so little you almost have to try and listen for it.
So it is possible to hook a bluetooth speaker to your sony headunit. I would assume most newer, or slightly older Airstream headunits will have similar outputs if you don't have a 2012. You could also splice into your LH and RH speaker wire and connect to the 3.5mm headphone jack on the micca as a last ditch effort, but that is a little wonky.
Josh