Welcome to the forum.
I can't comment on the Coachmen as I know nothing about the new ones.
Pertaining to the generator, however, I don't think you have choice any longer. I remember when Forest River first was making the transition from using Onan to NPS, they gave you a choice for a small window of time when the transition was being made. From what I'm understanding, every model of Forest River (including Coachmen) now use NPS generators at this point.
It's been reported that it's not actually a Yamaha product and they don't really have any association with the product other than they supply the engine (it's a Yamaha MZ360KHIP3) but the generator itself is manufactured by NPS and they are solely responsible for supporting the product. Yamaha generators themselves have the reputation of producing some of the best generators in the world. The company, of course, is a huge Japanese conglomerate but I've heard that a lot of parts come and some assembly is done in China. However, my understanding is that Yamaha has its own facilities in China and does not contract work out. The manufacturing is closely monitored by Yamaha as it is taking place in its own plants there. However, Yamaha also make some engines in Thailand and Vietnam and possibly in India too so parts or the engine itself could come from any one of those countries but again, under the close manufacturing supervision of Yamaha managers.
NPS is the actual manufacturer of the generator and they are relatively new in the business of RV generators even though they've been in the industrial power business for quite a few years. The RV generator operations are based in the Elkhart area, however. And again, they are now the exclusive provider of on-board generators for all Forest River products including the Coachmen line.
From all that I can gather, the two models they manufacture for on-board RV use sound like a very good products. Some are saying that they are quieter, more fuel efficient, and have cleaner emissions than a comparable Onan model. I'm not sure myself but let's see if others dispute this assertion.
NPS 4500/5500 Inverter Technology - powered by Yamaha