What the other poster improperly called "The converter panel" is in fact the Fuse box. the panel with the 12 volt fuses on it. there may or may not be a converter there, the proper name is 12 volt distribution panel but we normally just call it the fuse block, panel or box.
I agree with the "I would not tie to the 120 volt ground" you could end up in a very bad way doing that. The 120 volt ground often sits at 60 VAC believe it or not (I can explain why but it's not important).
Better to find a chassis ground spot if you have to.