This is looking like both AUTOMOTIVE WIRING and TRAILER WIRING worlds are being mixed together here.
The BATTERY CONNECTIONs are used in both worlds.
In the AUTOMOTIVE WORLD the dominant colors are RED for POSITIVE and BLACK for FRAME GROUND. This is what most of us OLD TIMERS are used to seeing.
In the RV TRAILER WORLD the dominant colors are BLACK for POSITIVE and WHITE for FRAME GROUND which is an attempt to follow standard HOUSEWIRING color schemes..
When both color schemes are being used in the same installation things will look very deceiving...
In the RV WORLD most factory wiring is BLACK for POSITIVE and WHITE for GROUND and then they add a JENSEN AUTOMOTIVE AM/FM RADIO which comes out of the box with RED for POSITIVE and BLACK for GROUND. Right away you now have a RED CABLE on top of a BLACK cable at the 12VDC distribution terminals. That is the way my JENSEN RADIO is wired at any rate... I suspect when you finally trace out your small BLACK WIRE that is connected to the RED CABLE you will find it does not go to frame ground. This is probably going to the PLUNGER BOX for the ELECTRIC BRAKEs wiring to power up the Electric brakes in the event the trailer becomes disconnected from the tow vehicles when being pulled.
Like said above if this use of both AUTOMOTIVE and RV TRAILER color schemes is present throughout the trailer wiring you have a mess on your hands... Hopefully in your case it is only around the BATTERY connections.
If you are planning on using more than one battery then I would really look into replacing all of the BATTERY WIRING from the converter/charger unit to the battery terminals as well as the Battery FRAME GROUND connections. This is the best time to install 4AWG cable size and then you won't have to worry much when you install more batteries later.
As it turns out in my case all of my inside TRAILER wiring except the JENSEN AM/FM radio and the BATTERY BANK location uses BLACK for POSITIVE (+) 12VDC and WHITE for NEGATIVE (-) 12VDC wiring. My inside 12VDC wiring uses a 2-conductor marine grade cable called BOAT CABLE which has 14AGW or 12AWG BLACK and WHITE color multi-strands conductors... Both POSITIVE and NEGATIVE cables go to where they are needed which does not depend on using any FRAME GROUND connection.
My TRAILER SAFETY lights coming from the 7-pin truck connector follow this same 2-conductor wiring scheme going to all of the lights. ALot of the older trailer only used ONE POSITIVE (+) 12VDC conductor to each safety light and the NEGATIVE LEAD (-) went to frame ground.
My Electric BRAKE wiring only uses a single conductor wiring from the 7-way junction box for the POSITIVE +12VDC and frame ground is used to return the NEGATIVE -12VDC connection from the ELECTRIC BRAKE MAGNETS back to the 7-WAY Trailer connection junction Box. The GROUND TERMINAL coming from the Truck 7-way connector goes straight to FRAME GROUND on the trailer side.
The cables from my CONVERTER/CHARGER unit 'BAT' Connections follow the AUTOMOTIVE WORLD COLOR SCHEME using RED for POSITIVE (+) and BLACK for NEGATIVE (-)... All of the wiring on my 2008 OFF-ROAD POPUP trailer around my BATTERY BANKS uses RED and BLACK wiring color scheme that I installed.
Just my shared comments here on how my OFF-ROAD POPUP trailer is wired... Other trailers are obviously wired different.
Roy Ken
