I doubt a cable actually stretched as it would take a huge amount of force to do that. It's more likely it popped off a pulley or moved where it is mounted at either end. In any case, I don't think it would be a "large" cost to repair it unless a major component actually broke and had to be replaced. That also depends on your definition of "large". Just a wild guess it would maybe cost $100-$200 to repair?
As a side note, I looked at the Norco Accuslide online service manual and I am impressed with the detail, photos, and repair procedure description. It seems far better than you usually get in the RV industry. If you take it back to the dealer mention the great online service manual so they are aware of it. It might seem ridiculous that a dealer wouldn't have the proper service instructions, but based on my contact with some dealers its entirely possible. Some dealer service guys just "dig in" without proper checking of the manufacturers service info and then use up a lot of hours($$) learning how to fix it the hard way.
Steve