If you're adding additional lines (outside your trailers already existing wiring), all you need to do is run the wire and just put a normal 15 / 20 amp household (3 pronged) plug so you can plug in a normal extension cord from the electric source to the trailer.
In my garage (house) I have run several additional additional plugs. I ran the wire to the desired locations, attached a normal box and plug, screwed it to a stud, and attached them all in a line. The beginning of the string just plugs into an existing plug with a normal plug. Good thing is, if I ever want to change it, all I have to do is unplug one plug, pull the wire off the wall, unscrew the boxes, and it's all gone! Simple.
Think of it as an elaborate extension cord with a plug on the end under your camper. Very simple to do. (or you could always just throw an extension cord through a window. )