I assume you have the XM series inverter? Per the manual, yellow light means it's running on inverter power. If you're connected to shore power, it should be green. Manual also confirms shore power should pass through the inverter even if the inverter is off.
It sounds like your inverter is not getting shore power for some reason and running off inverter power all the time. There should be an AC breaker for the inverter shore power with the rest of your AC breakers, likely in your DC converter panel. Make sure that is on and not tripped. If that is good, check the AC wiring on the inverter panel. Use caution, you're dealing with the 120v system, not the low voltage 12v system.
http://www.xantrex.com/power-product...inverters.aspx