SJ will work. It's nice and flexible.
4x4FF wrote:
It would just be extended from another outlet...
Check the outlet you want to "tap into". The outlets in our trailers are frequently thin, flemsy and have "push on" terminals that bite through the insulation and into the wire. That's fine (and cheap) for the original installation, but aggarvating for an "add on". Those type terminals also don't work well with an SJ type wire which has stranded conductors.
It may be easier for you to interrupt an AC line somewhere close and install a junction box and use good old wire nuts or other connections. It was for me.
On edit -
I'll either replace it with a box and standard outlet or just...
oops... didn't see your last post on changing the outlet before. Good move (make sure it will fit).