When the generator is running, or on shore power, you indicator is showing the DC volts from the AC to DC Converter. When the engine is running your indicator is showing engine alternator DC volts which is being supplied to the house to maintain a battery charge when going down the road.
I suspect the house battery disconnect switch is is active and has disconnect the batteries from the circuit. Normally the switch is used to preserve the battery charge when the coach is being stored and charge power is not available. Not sure where Winnebago installed the switch panel. Usually it will be near the door and have two momentary switches and two small lights to indicate if the circuit has power. One for the house and one for the chassis. If my suspicions are correct once the batteries are back in the circuit the solar will work as expected to maintain the battery charge when sufficient sunlight is available.