Based upon the virtually non-existent voltage drop while attempting to crank, three possibilities immediately come to mind:
1) Generator not properly grounded, therefore no current flow but good voltage at input,
2) Bad starter motor solenoid (i.e., faulty internal contacts), or
3) Bad starter motor.
There are plenty of other potential issues, but I'd look carefully at your grounding since the generator was working when you took delivery and is still attempting to start…. well, making a noise at least. A long drive on a rough road can easily loosen up a marginal electrical connection. Could also be bad grounding on either the starter or starter solenoid as well.
