The "under voltage" occurs under load, when the inverter is trying to produce 120v power and cannot get enough 12v amps from the battery to do it. Checking the battery before the load occurs or after the inverter shuts down doesn't prove anything useful.
Since a Xantrex Pro is a minimum of a 1000W inverter, chances are you are powering something substantial and the inverter is trying to get a lot of amps from your battery. 1000W means it could pull as much as 90 amps, which would cause the voltage in any one or two battery system to drop precipitously and go below the inverters minimum (probably 10.5v).
So, first tell us what you are trying to power and what size battery(s) you have. And also what size wires are used to connect the inverter to the battery.
