The Itasca motorhome speakers I was referring to in my post above were installed back in the coach area by Winnebago, are pretty good size, and mounted within fairly large enclosures made-up of excess areas back in the cabinets.
Using the fade control in the stock RV radio that came installed by Winnebago, I reduce the cab speakers' output to zero so all sound from AUX sources comes out of only the speakers back in the coach area.