There must have been an appraisal done in 2005 when you bought the home...Have you discussed the value with the original appraiser? Maybe appraisal rules may have changed in your state. In can say that by law ON FRAME modulars are appraised using ONLY manufactured home comparable in SC and GA..As a matter of fact there is NO on frame modular in GA anymore but older models are appraised using manufactured home comps..
All this means your home is compared with other similar manufactured homes in the area and their resale prices. At one time appraisers were aggressive in values but since the mortgage crisis they have been much more conservative especially since many appraisers have been charged with federal crimes for their past appraisals.
Sounds like you are a victim of the times...Take care of your home, enjoy it and expect to stay in it a while..