I'm not clear what redoing the electrical in a PUP has to do with removing a roof and replacing a 12' board? :h
I've removed the roof of a pop-up. The roof is not that hard to remove. The roof itself is held on by 4 bolts attached to the roof supports. Before you do that you have to remove the canvas. The canvas is held in with a plastic cord sewn inside the edge of the canvas that slides inside a channel at the top inside of the roof. If I recall there is a zipper or Velcro where the joint of the canvas is. You have to slide it out and take all the canvas off. Also there is probably wiring for the roof lights and AC that feeds through a channel in the canvas. You'll need 4 people to lift off the roof and put it back on later.
Of course if you have AC this slightly complicates things, mine did not have AC.
Once you get the roof off you'll see that replacing the board is pretty easy. Check with your local lumber yard, no reason you can't get a 12' board.
I wish you the best on this!