Xantrex guru please

Unless you have an external shunt and a battery monitoring device the bar graph on the display is largely voltage based. I don't know what all parameters Xantrex is reading to come up with the battery level but voltage is a huge component. That means, it's not terribly accurate. If you really want a battery gauge Xantrex does make one that works with their XW and SW products. I think it would likely work with your inverter as well but you'd have to check with them.
Otherwise, Trimetric makes a good meter and their are others. If it doesn't have a shunt, it's not really a battery gauge.