I have an RV travel trailer that I use a 2000W generator to supply power to the 30A incoming power socket. I have also added 2 × 100W solar with battery minder, a 3000W sine wave corrected inverter for electrical devices and replaced the original single 12V deep cycle with a pair of 6V golf cart batteries in series. The trailer has conventional (for an RV) electrical/propane fridge and is fitted with multiple 110V outlets for use when connected to the generator or municipal power. I've added a couple of (completely isolated) 110V outlets that are supplied by the battery bank through the inverter.
During sunny days, the solar is more than enough to keep everything happy; in fact, there is potential power being 'left on the table'. I want to add to my battery bank so as to take advantage of this but weight distribution (and trailer tongue weight) would dictate that I mount any new batteries to the rear luggage rack.
During my previous additions to the electrical system I have noticed that all 12V seems to be wired directly; i.e. one positive and one negative; the frame is not used as a common ground like an automobile. I was planning on paralleling additional batteries to the originals with 4 or 6 gauge AWS which can start getting expensive running the length of a 28ft trailer.
Is there any reason not to use the frame as the ground and only pass a single wire the length of the trailer?
Addendum:
The 3000W inverter was intentional overkill. The 3000W model had dual cooling fans, wattage and voltage metering, remote shut-off and two 12V ± inputs (two +, two -) on a bus bar. Smaller wattage models did not have all of these. While the parasitic power is slightly more than the smaller models it is a trade-off I can live with. I typically run 30-40W (ecostar TV and a tablet PC for a movie) although I have run an 8 amp (110V) wet/dry vacuum with no problem from an outlet powered by the inverter.
Best Answer
The main thing you'll need to worry about is the very large currents flowing. A 3000W inverter on a 12V battery will draw up to 250A. That means you need very low resistance wiring, and really good connections on all wire joins.
Bolting a lug onto a steel chassis may not be good enough in the long run, especially if the steel starts to rust around the join.
You'll want to keep the distance between the batteries and the inverter as short as possible, since the very large currents noted above will require very thick wires.