Welcome to the forums and congrats on your new Montana. Sorry to hear about your cable slide. It happens, you know. You have the tools at hand, now, just a bit of common sense and you'll have it fixed in no time.
For what it's worth, I just went through this myself. Here's a recent thread that covers my own repair, trials and tribulations with a broken cable.
Click Here.
As this was the first one I actually did myself, alone, I realize I made one mistake. Let me pass that on to you so you won't do the same.
After the cable was fed through all the pully's and everything in place, I put that little crimper in place and "thought" I did right. I placed the screw and the threaded bolt that hold it in position, about half-way in the bolt. I did not take into account that by looping the cable, it would need to stretch out and that loop would flatten out, causing the cable to have slack. So, after a couple of in's and out's with the slide, I realized the cable had slack. So, I tightened that end nut again. ... and again ... and again ... I'm now almost out of threaded bolt and still need to go even farther. My solution is to pack an inch or two of washers on the outside of the fixture, so the threaded nut will have more room.
So, if doing this, I strongly suggest you put that nut as far to the end as you can before doing the crimping. Once crimped, you'll be turning that nut inward a good inch, maybe even a good 2 inches until that cable bends itself into position and stays there.
The theory behind these cable slides is really very simple. And actually replacing them is very simple too. The problem and challenge is the very, very limited space to do the work. That's what makes them so challenging!
Good luck and once again 'Welcome!"