Joanne -
What school district?
I am just north of you, Newaygo, Hardy Dam area, and I can tell you the manufactured housing market is very volatile right now. There is an oversupply of double wide manufactured houses in almost every market, and a lot of foreclosure sales.
No, doublewides should not be used for single wides in our area. The only time I have ever used a double wide for a single wide was when the single wide had a large site built addition, and then the double wide I used was an older one, and I only utilized one in order to bracket some amenities.
Being you are in the Kent City area, I can tell you that if the subject is in Kent City, there is currently an oversupply of available double wides in the Kent City area. There are no sales of single wides there that I recall (just did a report in the area), however, I would go to surrounding school districts for the single wide as primary motivation is the affordable housing. I would certainly have looked at Sparta, Cedar Springs, Tri-County, and Grant School District areas, surely there are single wide sales there.
Single wides have a larger foreclosure percentage in this area than double wides, but, double wides may catch up if the economy around here doesn't change. Also, single wides typically have the aluminum skirting versus the concrete block skirting of double wides - bedroom sizes are minimal, and functional adequacy is reduced.
Fannie Mae requires that the appraiser be experienced in the appraisal of manufactured houses, and that at least 2 similar manufactured houses are utilized in the sales comparison analysis. Sounds like your appraiser missed the boat there.
Kudos to you for sending the appraiser to look at the improvements. It would appear you have a skippy on your hands, or a very inexperienced appraiser.