All of your 12 Volt circuits have fuses of varying capacity. The load on each circuit has been calculated to not exceed the fuse rating. Adding extra devices to a circuit could exceed the fuse's capacity and cause you to start "blowing" fuses. Consequently, you need to carefully evaluate the draw of any device before you add it to an existing circuit. For example: if you want to add an extra LED light to your lighting circuit that would not be an issue because the LED light draws so little power that it is not likely to overload your fuse, on the other hand, if you wanted to add a 12v fan to the light circuit you could easily overload the circuit and "Blow" the fuse.

The best way to add Extra 12v outlets, because the load of what gets plugged into the outlet can't be predetermined, is to add a new FUSED circuit for the outlet(s). This is easily accomplished by adding a "In-Line" fuse to the new circuit (positive wire).

As far as what to use to fish the wires throughout the trailer, you can use a combination of "Fish Tape" and stiff wire. I would recommend using the nylon fish tape instead of the stiff wire type for the trailer. There are small 10 foot fish tapes available which would be useful in the compact space of your trailer. Also, I like to use the 12 ga copper wire from a short length of Romex (house wiring) to supplement and/or instead of my fish tape. I often find it useful to form a hook on the end of the wire to "snag" the fish tape from the opposite end of the access point.