I extended an outlet from the outlet at the head of our Murphy bed to the front of the side table. There was one like it on the side table on my side of the bed. The DW wanted one on her side.
Be aware that many RV outlets don't attach the wiring with screws, but have slotted blades that the wires slide into, cutting the insulation and creating the connection. They're called "speed wiring" outlets. The installers don't have to strip the wires and screw them to the outlet; they just lay the wires into the top of the slotted blades and press the back cover on. A little hard to see, but check it out here:
You can see one of the slotted blades right behind the "t" in "store" in the pic.
So I had to open up the existing RV outlet, add another set of wires from the other end of the outlet (each slotted blade can take 2 wires), then route that wire to my new box and outlet. I used a standard house outlet with screws for the wires.
Make sure you wire it correctly, with the hot wire on the proper screws. If you don't have one of these, get one and be sure:
Ge 50542 Receptacle Tester - Walmart.com
Be careful, though. If you're going to be putting a lot of load on the circuit, you might be better off adding another breaker, if your breaker box has a spare slot, and running a separate line to the new outlet.