The trailers are built to operate from both 120VAC and 12VDC batteries.
The concept is to use the 120VAC coming from shore power or a connected 120VAC Generator for the high wattage appliances and use the 12VDC for the very basic items such as ceiling lights, detectors, small wattage items, etc...
When you plug the trailer into 120VAC then the on-board converter gets 120VAC power to keep your battery charged as well as run the 12VDC power distribution panel.
This is neat electrical block diagram for a typical 30AM Trailer setup...
Do some reading up on line how the RV TRAILERS are suppose to work before tearing into everything. You will soon as a big mess on hands tearing it all apart...
Just install a good battery and your ceiling and other lights should work off of the battery.
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