The Appraiser must include a sufficient number of sales to produce a credible value. At least two of the comparable sales properties must be Manufactured Housing.
Instructions detailing specific requirements for reporting the results of the appraisal are found in the FHA Single Family Housing Appraisal Report and Data Delivery Guide at https://www.hud.gov/program_offices/housing/sfh/handbook_references
For additional information see Handbook 4000.1 II.D.5.k. at https://www.hud.gov/program_offices/administration/hudclips/handbooks/hsgh