it's better to build on a spartan or freightliner chassis

It all depends on whether it's a class-C type cutaway where the cab is built by the chassis manufacturer (so you get a steel cab plus airbags and other safety features), or a class-A built on a stripped chassis where everything but the frame and engine comes from the RV manufacturer -- many of those are essentially a lightweight box on top of the frame and will disintegrate in a crash, and you'll likely not survive a rollover crash. Even with a cutaway chassis, depending on how much of the cab they cutover for cab access, you may not fare too well, but much better than with nothing but a wood and fiberglass box surrounding you.

With a cutaway, the manufacturer takes care of all of the steering linkages, driving controls, etc so you have some assurance of quality -- everything related to driving is built by the company with the most experience in building trucks and cars rather than the RV manufacturer.

I don't know what you get in the $300K+ Newmars and other expensive rigs since that's way out of my (and most people's) price range.