Manufactured homes settle in time as does anything else. It would be suspected that a home were out of level if the drywall begins to crack, windows and doors become difficult to operate and don't lock or latch properly, worsening floor squeaks, and cakes coming out of the oven lop sided. When installing tie downs or another retrofit system which are designed to hold a house down it is a good idea to relevel first. It is suggested that this be done every 3-5 years depending on the soil conditions. Metal piers which have rusted should be replaced.