I've posted on this before. Here's what I did to strengthen the front slide:
https://www.montanaowners.com/forums...10&postcount=2
The picture below is the top of the vertical angle spline I added that goes from the bracket that bolts to the frame all the way up the side. I had to cut one leg down to clear some of the interior trim when the slide is out.
It's been a 1.5 years and has about 3000 miles since I repaired. No movement at all. I have the Reese Goosebox which adds a little more movement and I also double tow my jet skis.