Searching for 'ground fault' on DC power supply, 0VDC to ground
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Earth fault tester.

Try wiring two 120 V lamps as shown in Figure 1. If the system is floating both lamps will light at half brightness. If one lights more brightly it indicates an earth fault on the other line.

Retake your voltage readings with the lamps connected. Their lower impedance will give a more reliable "pull" than your high impedance meter.