FWIW, I did extensive rewiring in our 2009 FC. From this I can tell you that Airstream direct wires all 12V
DC devices back to the ground bus. They also install a ground post on each major section of the body to which they run a 10 gauge 12V
ground wire (white insulation).
So I would follow Lewster's advice above and ground thru wiring. Yes, the frame and its sub components are grounded but Airstream, over the years, has found it cheaper, in fielding customer warranty claims, to invest in ground wires instead of relying on chassis grounds.