Haven't looked at the diagram but I believe the last wire might be for backup lights. Our MH has a 7 wire plug and brake controller installed. Previous owner said he never towed with it. I was blowing turn indicator/backup light fuse when the towed was hooked up. Same as yours, 1 white wire, 5 yellow, plus one from the brake controller. Several of the wires were in the wrong spot, backup light wire was connected where the ground should have been. Anyway, I think that's what your mystery wire is for and you really don't need it.
Hank & Lynda
2003 Winnebago Adventurer 35U