Did you also measure AC voltage between GND and minus ?
The connection to the voltages, if it were AC, might be explained by the output common-mode filter capacitors being a voltage divider with center going to GND. There would not be much if any possible high current behind it though. You could verify this by taking a low to mid Ohm resistor and tying that between the negative and GND and then see what the AC or DC voltage is.
It might also be part of a DC side filter section ?
I am not sure what a Xantrex XW 6748 pro is though ? 6.7 kW 48V inverter ?
boB