If the trailer already has 14-2, you'll have better results keeping the same size wire if you plan to "jump" from an existing outlet to a new one. RV outlets usually are connected to the wire with a split metal piece (not a screw like household outlets) and the wire is shoved into the split which cuts the insulation and makes the electrical connection. If you jam a 12 wire into a connection that already has a 14 wire in it, the connection to the 14 wire will loosen and possibly arc/overheat. I found this my Rockwood 2604WS when I added 2 outlets inside the camper.
2015 Rockwood Ultra Lite 2604 WS. Rear LR, large street side slide with small wardrobe slide in BR.
2014 Ram 1500 Eco Diesel Laramie Quad Cab.