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?
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.