You need to find dealer or mobile rv repairman that has the proper equipment to handle something as large as a slide-out, especially a full wall slide.
You can google the problem and read the results to give some kind of idea of what to expect.
This quote is from someone who fixed his on a Monaco MH-
"after further investigation i realized that the front roller that supports the slideout is just screwed into the plywood floor. I removed the rollers and fabricated a steel bracket that I welded to the existing cross bars of the frame. now the roller assembly bolts to the steel bracket rather than the floor. solved the sagging and after a few minutes of adjusting the other rollers i had the slide out working smoothly and sealing completly. "