Possible to daisy chain bluetooth speakers on auto-stereo?

Jabra Bluetooth makes an adapter that will work for your need. I've used mine in conjunction with the RV radio. Although this is designed for cell phones, it will work on other gadgets. The output plug on the bluetooth adapter would be the input signal to your RV radio.
The bottomline is: Use the output signal from the adapter and use this signal to the pre-amp circuit of the RV radio--thereby using the volume control of your radio.

Bluetooth Adapter Jabra A210. I don't have the website but try googling it.
Good Luck