You can't use the NADA book to make market derived quality adjustments (or any other types of adjustments.) Same with M&S or any other cost service. It's cost data not market data. NADA is an amalgam of numerous data sources. It's based on MSRP, sales to dealers, retailer sales statistics, sales from rental parks, consignment sales, insurance companies, market data research, etc., etc., etc. It's a system developed over many years for valuing not just manufactured hosues, but vehicles, RV's, commercial buildings, boats, motorcycles, buses, etc., etc.
But obviously the market will react to quality. They will pay more for high quality and less for low quality. High quality costs more and low quality costs less.
The difficulty, IMO, is that there are always far fewer sales of MH's than of site built homes and other value influencing characteristics usually eclipse the the single factor of quality. General market noise is usually enough to erase a really supportable adjustment for quality differences. The general ignorance and laziness of listing agents is another big factor because they seldom include enough information in their listings.
But if your market has enough regular sales of manufactured homes and you regularly appraise manufactured housing over a long period of time, have made it a priority and commitment to learn all there is to learn about manufacttured housing, and routinely stick to honest, detailed and well documented cost approaches every single time you appraise this property type, you will easily spot trends related to market reaction to quality differences.
You must not pull a number from NADA and stick it into the adjustment line, but you can use the data to help corroborate and support an opinion. Even though I've never been asked to explain an appraisal or an adjustment, most of the time if I need to make an age or quality adjustment and I know or have a good idea of the model or quality (and can find a similar model in NADA) I will run a quickie cost sheet from the CD version of NADA for that comps and stick the printout in my work file.
This is where I'm at now. Maybe as I learn more over the years I'll come up with something different.