If the heavy wire leads in the general direction of the generator, that's a very likely destination for it. Generators are commonly connected to the house 12V system, and the generator starter motor uses a lot of current when cranking and so needs a heavy wire. 2 gauge would not be out of line. (My Onan 4KY generator uses somewhere in the rough vicinity of 100A when cranking.) RV generators also need a little bit of 12V power to generate, to power the control electronics and fuel pump and so forth.
There likely is also be a heavy wire going off to the chassis electrical system via an isolator of some sort to charge the house battery from the alternator while en route. It's possible that the same wire would be used for both.
I would urge some caution about relying on wire color to know which wires are positive and which are ground. It's nice when they're reasonable and consistent, but it's far from universal, particularly after various owners and repairmen have tinkered with things.
I don't know where your ground lead went from the factory. On my (completely different) motorhome, it just goes from the battery to a nearby bolt and nut on a chassis frame member. It was quite plainly obvious. The wire itself needed replacement when I got the RV (the connector was broken), but the routing to ground was in plain sight.
