1st mark about 24" away from the outside grommet. Coat the 4" of wire a a time lightly with some kind of lubricant . Have someone pull the wire from the inside while you push it in the grommet from the outside. Can you get it through with some effort? If so continue reading.
I've done this with my son's SOB. Cut the old wire about 2' before the entrance grommet. Strip the wire down to the copper about 8-10 inches. Cut away about 1/2 half of the copper wires (Thin it out). This is so the wire isn't thicker when braided together. Do the same to the new cord. Then twist them together in 3 sets. Kind of like braids, not as if you were going to put on a wire nut. For added strength you could even solder them together. wrap with tape to further strengthen. lubricate the old wire hanging out, the splice, & as much of the new wire as needed to make it reach it's new home. Use the old wire to pull the new wire in.
"Sometimes I wonder if the world is being run by smart people who are putting us on, or by imbeciles who really mean it." Mark Twain
AIR 22749 WBCCI 2349 NOVA TAC TN-6
1989 345 LE Classic Motorhome