Anyone fished 12v wires through the ceiling joists?

I just did this exact thing. First check what path said, then see where the closest fan is, because you might be able to thief off those wires.
I've used a fishing pole type of device, or heavy gauge wire to poke around. On my trailer, the trusses had more room in the center where there is a crown in the roof. Insulation was a problem and it was hard to move it around. Your project might be a piece of cake.