You'd think dealers, and especially Winnebago, would know all about the products they sell. But that's certainly not the case for the dealer. And, Winnebago? If you talk to the right person at WBGO they could tell you. But otherwise I'm not surprised.
The wiring diagram SHOULD show the extra speakers but I don't see any listed.
You could get one of those wire tracer kits that has a sender you connect to one wire and then you use a small beep locator to look for hidden wires. I realize that's a bit much. But if you really want to know... maybe not.
https://www.amazon.com/dp/B00ADHQCIO/