As one other situation, our new 2005 Class C was a built on a Ford 2005 chassis, so it appears that all was in order. However, Ford actually constructed our particular chassis in the fall of 2004 ... obviously they built it ahead of calendar 2005, but per their advertised "2005 chassis design and features". Of course all vehicles of any given year that one buys early in that year are likely to actually have been put together in the previous year.
IMHO, the important thing with the OP's situation is ... if their chassis was truely of the previous year's design ... then what, if any, improvement changes did Ford make from the 2014 chassis to the 2015 chassis? If none of any significance, then I wouldn't be concerned if the RV is called the year model in which the coach portion meets the advertised design and features of that year for that coach, even though the Ford chassis only meets Ford's advertised chassis design and features of the previous year.