No disagreement about the need for a run back to battery negative, even a 600 watt inverter means over 60 peak amps possible, so one needs to have something beefy enough to conduct with minimal loss.
Still trying to understand the ground thing, though. A ground is to give an alternate path for the current, so you don't become the primary. But it needs to have a good path - either a ground rod, or the steel of a building, to get into the real ground. Hence the NEC rules. In an RV (or car) the structure also serves as the negative run for the rest of the 12v. So does it matter if the ground goes direct to the structure, or follows the wire to the battery terminal and then to the structure...?