We are talking about a couple of different issues. Running a Honda 2000i next to anyone's model mounted in an RV is a huge difference, but I have yet to see someone with a large output (4000+) that is "whisper quiet" like the Honda 2000's. Run the Honda in a tent campground and people might look the other way. Run ANY 4000 in the same conditions and people will be asking when you are turning it off.
That said, if you look up an Onan QG 7000, it shows a noise rating of 70 db. Look at the HF equivalent model (8500) and it shows 76 db. That would be noticeable, but not huge. Also notice that the Onan is a fully encased design and runs you $5000. I very much trust an Onan genny, but I would have a VERY hard time justifying the $4500 price difference. Especially when folks like Consumer Reports list the HF genny as a top of its class purchase.
From where I sit, if you can mount the HF genny inside your RV, it most likely has a properly insulated generator compartment and noise will probably not be an issue. Adding a sound shield to bring it down to the same noise level as the encased Onan would probably not be very difficult either.
The one thing to sort out is the fuel supply. The HF genny has a gravity feed carb, so you might need to do something special about that. Considering the $4500 price difference compared to the Onan, that seems like a very small inconvenience.
Jose