Trailer wiring is not in the National Electric Code but most of the time you need to use the basics like in your home.
On adding your outlets I would take one out and see what they used in the factory. I would suspect it is stranded and would be very surprised if it is solid.
On any 15 amp ciruit you should use #14 awg copper wire. I would also suggest you make sure you ground all the receptacles. If outlets are on the outside of the rig or in cargo areas they need to be GFI protected.
That's all I can tell you other than the black is HOT, WHITE is NOT and the green is Ground. Oh the hot wire always goes on the copper terminal screws and the white to the silver terminal screws.
DID YOU KNOW ELECTRICIANS DO IT WITHOUT SHORTS?