Here is what I would recommend which is fairly close to what Airstream seems to be doing on current production.
1) All connections should be made using solder and heatshrink tubing, including the connections to the magnets. I have never seen a properly soldered 12 volt
connection fail. Strip the wires about 2 inches, twist, and solder the 2" or so length at each junction.
1a) If you do use crimp on connectors be sure they are in an area protected from road spray and crimp them with the proper tool not some $20 piece of junk from harbor freight
2) Whether you wire in what is called "home run" or sometimes "multiple" fashion, where each brake has its own pair of wires going to the umbilical, or "parallel" fashion, where the wires are sort of daisy chained from one magnet to the next, is immaterial.
3) It does not matter where the connections go because you soldering and heatshrinking the connections and therefore you will never have to look at them again.
4) You should plan on wiring the hot and ground the same way instead of depending on the frame to carry some of the current. The problem is that a grounding connection to the frame may fail due to corrosion while a wired connection generally will not except from mechanical damage.
5) Put the wires inside something. Flexible "loom" is fine. Plastic conduit is fine. Leftover pex is fine. Anything that provides a degree of protection from the abrasive effects of rocks and other debris.