Good thing you checked that out. All very mysterious. You still have the RV battery neg grounded, but the existing set-up must allow for that somehow. Perhaps being "internally positive grounded" means they are not "externally" grounded to the RV so the battery being neg grounded doesn't matter. No idea about the controller.
So what will you do? Parallel or series the second panel (also internally pos grounded?) to the existing controller and battery or just what?
Lots of people have those Grape solar panels, but this topic has not come up. Any more info on all this would be interesting.