Tony, first of all welcome to NewellClassic.com.
Glad you are searching and seeking out a quality coach. For the most part, you should obtain great information here in the forum.
This topic has been addressed on the forum a couple of times. I know for one, I could write for hours on the differences between a “bus conversion” and a “Newell Coach”. A bus is that, a bus! And a Newell Coach is a ground up designed, engineered and built motorcoach or motorhome. At Newell Coach, they engineer and integrate all systems to function together for their designed purpose. I for one, like many other members here, have been down the same road of research it sounds as if you are on. I actually have a history with Marathon and respect the company very much. They build a great conversion and exactly that, a conversion. Myself, I can’t find a real comparison between a converted bus and a factory built Newell Coach. There have been some improvements with companies such as Prevost and MCI building shells with slides for converters like the ones you mentioned. But, they are still a conversion.
Best of luck to you and hope you too find all the greatness in a Newell. Please take advantage of the information here. I’m sure many will add their opinions and thoughts.
Ken W.