Scrubs one side of the wall and a gap on the other side
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Make sure there's nothing stuck top or bottom or at the end that has a gap.
Check under the slide. There's an adjusting nut, with a jam nut. Together they lock the adjustment. If one of the nuts is loose, the adjustment will change.
Look from front to rear along the outside bottom edge of the slide; in particular where the brackets attach to the outside bottom wall of the slide room to make sure nothing is twisted.
If you don't find anything, take a picture of each side that clearly shows one side in all the way and the other side having a gap. Call Heartland back and ask them for possible causes.
Btw, the owners manual has zero references to slideouts in the maintenance schedule, and the only reference in the slideout section to owner maintenance is periodic lubrication of the rails and care of the seals. There is no intended periodic adjustment.
You might also ask what periodic maintenance, like an oil change, would avoid or correct the problem you're having. In other words, if there's an adjustment you're responsible for, ask for the exact directions on what needs to be adjusted.