I think fishing a line in the wall is going to be pretty close to impossible. You can see the construction of Sabres here:
Learn To RV: Palomino Sabre Factory Tour Photos
Specifically, here is the wall- it's aluminum framed with insulation filling the voids:
With your awning deployed- at the bottom of the post on the wall is a plastic cover that is pressure fit into place. When you lift that up, towards the bottom you'll see a big glob of caulk filling a hole through the wall where the power wires go through.
I pulled that and was able to find the corresponding spot inside where it was (very close to the floor/door of the front bedroom). Because I have the washer/dryer hookups in the front closet, there is a raised part of the floor for the drain pipe. That is open on the end that mates up against the dresser in the front bedroom.
I was able to push my new wire through that hole with a loop at the end, then I used a piece of stronger wire to fashion a hook and was able to grab it and pull it. Using that same raised area, I was able to fish it down into the basement storage.