Describe what you mean by "loose". What year is the Landmark? For best results, please give as much information as possible. Pictures will help. When you indicated "Landmark say its normal", to you mean Heartland says it's normal? Who at Heartland?
When it was built that "skin" is glued to the steel framework to hold it up. Over time ours has some slack, or sagging in it as well. I would think the only way to put it back up, is to use screws to hold it instead of the glue.
If your are referring to the thin fiberglass curved skin, its sorta normal. On my first 5th, wheel this happened. I screwed the skin to the support members above it with stainless screws and automotive trim washers. I have seen this done on other units, both Heartland and SOB.