Different strokes for different folks. If someone gave me an Onan I would sell it. Others do not mind the expense or owning one. Box's for the gensets vary a bit but are basically a metal lined box.
Soundproofing is not a consideration in the RV industry, until getting up in the high end coach's. The older Onans are horribly noisy and because the way they are added to the RV are difficult to quiet down. The new ones may be quieter?? but unlikely, as the only online statement I see is the new LP ones are 15% quieter than a diesel generator.
You tube and the web will be your friend on this, there is lots of videos and info. Here is a sample. I use a window AC and a Honda 2000, not for everybody.
https://www.youtube.com/watch?v=s39PJtgZ7HA