I don’t think most buyers are concerned as much about the spread between the chassis and coach title year unless or until it becomes an issue.
If they did have a problem with the spread I presume they would not proceed with the purchase. The title controls the year the market sees as the coach year. If a coach has a spread of several years it could be of concern but I think most buyers will accept the year on the title as the coach age regardless of the chassis year.
There are true issues that knowledgible buyers recognize. If the spread in years is four years for example, especially if the chassis had significant changes that took place around the time shown on the title by the converter the coach may not have those features. Specifically, as an example Prevost went to a multiplex electrical system and had what are now called pinless slides around 2007. A chassis manufactured in 2005 would have neither feature, yet the coach could carry a 2008 or 2009 year on the title.
A similar problem may impact a warranty issue. I believe at one time Detroit honored the engine warranty from the date of the title, but changed their warranty coverage based on the date of engine manufacture. At best an engine is going to have a date of manufacture within the chassis model year, but it is also possible the date could be much earlier. A coach with a 4 year spread could have an engine with very little remaining engine warranty.
A lot of the coaches with greater than a one or two year spread were in the period when the economy was at its worst. I am not sure if any coaches built today will have a significant spread because very few if any spec coaches are being built.