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When the home is not level, the frame is not square and puts pressure on different areas of the home. Leveling relieves that pressure and most times will cure the squeaking.
It depends on the ground under and around the home. Soft ground can cause your home to become not level more often thank it would on concrete ribbons or compacted pads. It is best to have your home looked at every couple of years.
Underpinning keeps moisture away from the insulation and sub-flooring. Moisture will cause mold and rot and can result in more expensive and expansive repairs.
If your power bill has been inexplicably high, if your floor is soft, or if you see tears or hanging underpinning, your insulation may need to be replaced.
Hurricane straps are standard on most mobile homes but can rust and deteriorate. An Xi2 system is state and federally approved and consists of galvanized metal tubing that will never need to be replaced.
Tie-downs need to be tight with no drooping.