I generally stick with the same gauge wire that was used by the manufacturer, although it's not necessary - LEDs draw much less power than incandescent bulbs, & can easily run on much smaller gauge wire.
Since there's never anyone staying in the trailer other than my wife & me, we leave the sofa set up as a bed full time, & wanted better lighting overhead so we can read in bed. I mounted 3 "Good Earth" dimmable 12 volt LED "puck" lights (https://www.lowes.com/pd/Good-Earth-...ght/1000410931
) on the bottom of the upper cabinet on the street side, using the wiring that came in the light kit to wire them into the trailer's 12 volt system. Good Earth's wiring was very light - probably around 20 gauge (about the same size as the wire that usually comes with relatively inexpensive automotive speaker kits) - the lights have worked flawlessly for two years now.