It seems that you don't have AC power at the shore power terminals inside the inverter or the inverter has an internal problem. Yes, you should get a green LED if you turn on the inverter while on shore power. You should not have to turn on the inverter, when it is off it cuts the shore power thru to the inverter output side.
The output side goes thru the GFCI outlet in the inverter, if this trips all the outlets fed by the inverter will be dead until you press the GFCI in the inverter reset button, but if you are getting power to the output side of the inverter TVs, etc. when the Yellow light is on then that GFCI has not tripped.
First, check the circuit breakers in the power box and reset any that have tripped.
Second, if you have the skills to safely do so, you would remove a saftey cover on the inverter and check there for the proper shore power on the shore power terminals of the inverter. If there, then you have an internal problem in the inverter that needs repair. If not, then you have a problem with the branch circuit trying to feed power to the inverter. The inverter is an option and there are a couple pairs of 3M 120 VAC connectors in series with the input and output AC cables of the inverters, it's slightly possible there could be a bad connection in the input side connectors. The connectors are there so that Winnebago factory workers can quickly and safely break the branch circuit and insert the inverter for those coaches built with the inverter option.
Manual
Status LED Indicates the mode of operation with a three-color LED.
• Utility (green) indicates the XM 1800 is in shore power mode,
using utility power to operate loads connected to the XM 1800.
• Battery (yellow) indicates the XM 1800 is in inverter mode,
using the battery to provide AC power.
• Fault (red) indicates the XM 1800 has shut down because of a
fault. “Understanding Fault Codes” on page 5–2.
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