There are a number of different circuits that run through the ceiling. As a "GENERAL RULE" (not always gospel) the 120VAC is run as ROMEX, the 12VDC wiring is run as either coupled wires or single wiring runs. The 12 VDC wiring is generally not enclosed in a sheath so if you've found a red/white pair and a black/white pair, I'd suspect that both are 12VDC. If you've found 12 ga Romex, it's likely to be the air conditioner power supply, if you've found a 14 ga Romex, it's probably the wiring run for outlets somewhere in the trailer.
I hope this explanation "sort of" leads you in the right direction. If you're unsure, use a multimeter with a needle probe to check for voltage, but generally speaking, if it's a single wire, it's 12 VDC. And, yes, many people will tap power from the light source. If it's converted to LED, you'll probably have more than enough power available without overloading the wiring.
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