I have used that pre-wrapped 7 wire trailer RV wire. It is not real cheap, but it is really nice. Also, the trailers that we would be building are going to be built with 4x2 boxed tube and not angle iron. So the wires will all be ran inside the trailer frame. We also solder all of our connections and shrink tube.
I will put some pics of the trailer I am building a little later tonight.
keep the good ideas coming!