I used 8 AWG THNN wire from the battery in the front tongue box back to the fuse panel in the back galley, and 14 AWG THHN wires for all the run from the fuse box to each component (light, fan, 12 volt socket, etc). To avoid problems each component was wired separately from the fuse box and back. This allowed for splices to be isolated at the fuse box, battery, or the component itself in order to avoid potential issues in the future with broken connections within the walls that would be inaccessible. Black and red wires were used to help me keep hot and ground straight.
Aside from the 12 volt system, I had a separate system for the trailer lights (side indicator, tail, break), that had to be wired as well. These were wired similarly with individual runs to to each light that met back above the galley electrical compartment so all splices could be localized. From there a group of wires were then run to the front of the trailer into the tongue box.
First of all I ran all the wires to I would need to get the length for each run before I drilled any holes.
I glued in a backing plate so I could mount the cabin dome light.
Once I found out the lengths all all the wire runs I drilled the wiring channels through the roof spars. The left channel contains the main positive and ground wires from the battery, and on the right are the component wires.
Similarly the left wires are components and to the right are the trailer lights.
With the wires run I re-installed the insulation in the walls, and began installing the insulation in the roof. The roof has 2 layers of 3/4″ insulation with the wires running between them. The insulation was secured with PL 300 Foamboard adhesive.
Clamps and blocks were used to hold the insulation in place until the adhesive cured.
The second layer in installed facing outward, and channels were created to allow passage of the wires.