The 7-way trailer cord purpose is to operate your trailer safety lights and electric brakes if you have them installed. In some cases the 12VDC from the truck is available for connection to your trailer installed battery but not always the case. My truck has to have a relay and separate fuse installed in the truck fuse panel to engage trailer tow charge. My trailer tow charge from the truck is only active when the truck ignition switch is turned on.
My trailer 7-way cable is terminated into a metal junction box just under the frame of the trailer tongue area. All of the trailer lights, trailer emergency brakes quick-disconnect, and a 12VDC line to connect to the battery comes from this electrical junction box. The line that feeds the trailer battery has an in-line fuse in it.
This Has nothing to do with the trailer 12VDC system which is controlled by the Trailer 12VDC Power Distribution Panel. This 12VDc Distribution Panel is powered only the on-board converter/charger when plugged into shore power and is also powered by the trailer installed Battery setup.
This is a 30AMP electrical diagram that shows how all of this is setup that might be helpful for you to identify where things are.
This is a typical 7-way trailer wiring setup from ETRAILER.COM - Wiring colors may differ by the various manufactures.
Hope this helps...
Roy Ken
My Posts are IMHO based on my experiences - Words in CAPS does not mean I am shouting
Roy - Carolyn
RETIRED DOAF/DON/DOD/CONTR RADIO TECH (42yrs)
K9PHT (Since 1957) 146.52M
2010 F150, 5.4,3:73 Gears,SCab
2008 Starcraft 14RT EU2000i GEN
2005 Flagstaff 8528RESS