Neither of those have anything to do with signalling. And it's entirely possible to change the source of power from one RV fixture to another by swapping wires at the source.
I think I've said this recently on this forum or another, but you should really Google some electrical...
Then your trailer is wired incorrectly. There should be no fused wires between the trailer receptacle and the light fixtures themselves. The manufacturer is required by law to wire the trailer's signalling capabilities independent of its other 12v systems.
You have two cases with 6 cells each. All twelve cells in series is what is known as a "battery"
More importantly, the way you describe your troubleshooting methods scares the bejesus out of me. Please invest in an electrical meter and a couple of YouTube series on low voltage electrical...
noun, plural batteries.
Electricity.Also called galvanic battery, voltaic battery. a combination of two or morecell electrically connected to worktogether to produce electric energy.cell1(def 7a).
any large group or series of related things:
a battery of questions.