When plugged in or running the generator, the control panel is showing output voltage from the converter/charger. If the engine is running it could be showing voltage being supplied to the house from the engine alternator.
When not plugged in or running engine, it should show house battery voltage, but only if the house batteries are connected to the house 12V system. They are not connected when the "auxiliary battery" switch is off, or if the fuse or circuit breaker for those batteries is open. The place to measure battery voltage is at the battery posts, ideally disconnected because when connected together in parallel the reading you will get is from the battery with the better charge.
I don't know how your solar is wired, my Winnebago doesn't have solar. It might also be disconnected when the batteries are disconnected.