Interesting. Ours has the (worthless) Jensen in the dash, but it drives only the cab speakers. The entertainment center above the side entry door runs the house speakers, and has 3 outputs. We have two internal and two external speakers. The default when the center is initially powered up is to run all outputs, inside and out.
(Soapbox) I say the dash unit is worthless, as the GPS appears to have been designed by someone on a huge desktop monitor. When the map is reduced to the small display, it's almost completely unreadable. I like to use my dashtop Garmin anyway - it's much easier to use, and is consistent no matter what vehicle I put it in!
Additionally, the thing died a while back - no power whatsoever. We rarely used it anyway, so I haven't spent much time trying to troubleshoot it. All fuses are good, so it's an issue with the supply wiring or the unit itself. As the entire dash facade needs to be removed to access it on our GM, I probably won't bother until I find a decent replacement.(/Soapbox)
Formerly: 2000 Four Winds 26Q/Ford, '96 Kit Sportsmaster 22' 5th wheel, '91 Sunlite truck camper, Wildernest flip-top canopy