Lot's of information all over the map here. Much really has nothing to do with what you asked about bus conversion coaches. First off when you are talking Prevost/MCI or "bus conversion" coach these are on a completely different chassis design than any of the "box" motorhomes most of us own. They use a monocoque chassis which is built like an airplane fuselage. Very strong and stable. One drawback is at the rear of the coach it is a little more difficult to attach a traditional hitch for towing. The hitches designed by the coach/conversion company will usually entail a large structure that ties much of the rear of the coach together which is where the hitches strength comes from. The downside is that this usually will interfere with some maintenance procedures. The only production motorhomes that even come close to strength and durability would be the Foretravel and Newell coaches which both fabricate their own chassis and it is very similar to the Prevost in design. Some other production coaches are closer by using a semi-monocoque chassis design which incorporates two large framerails and a monocoque design for the house intergrated into the framerails running the entire length of the coach. Some of these are the upper end Country Coaches (Affinity and Magna I beleive), upper end Monaco coaches (Dynasty, Executive and Signature) and the upper end American coaches which use the Liberty chassis. Don't quote me on the Liberty chassis though.
On to the drivetrain side of things, the older coaches, pre-1995, will be the ones with the two-stroke Detroits. Although they were great in their day and still run strong they are old technology. Also very noisy compared to the Series 60 Detroits. They are not what one would call, environmentally friendly, therefore many states (CA in particular) are making it much harder for these coaches to get re-certified for on-road driving. It is very hard to get these two-stroke engines to pass an opacity test even when the parameters are in the high 60 to near 70 percent range. Even some of the old CAT 3208 engines are getting hard to meet the ever tightening regulations. I would hate to see you purchase something that is a PITA to get smogged and looked down upon each year when you try to license it. Another standing joke we have at work seems to be that one of the things Detroit did when they designed/implemented the Series 60 engine is that they actually used gaskets.:D The two-stroke Detroits leaked like sieves if you looked at them wrong.
I don't want to offend the 2-stroke lovers but personally I would not even consider a coach with a 2-stroke engine at this time with all of the regulations tightening up more and more each year.
The "body" design on these is usaully a glued/riveted stainless steel skin which in itself is very durable and adds to the strength of the monocoque chassis. There were a few years in the Prevost line that had some bonding/disbonding issues but this will require some investigation as I am not certain on the years/models and I don't want to throw out erroneous information. They were having some problems with the skin disbonding and then some corrosion/rust forming on the chassis itself. Again, I would sign up on the Prevost forum and get correct information on this as I don't want to misquote or misrepresent.
As for what you are looking for overall the durability and quality is steps above most anything else most of us on the forum have. That said, as with everything there are pros and cons. One of the biggest cons of these Prevost/MCI bus conversions is weight. They are heavy, no discounting that. The components are heavy truck parts and very readily available so nothing is really proprietary drivetrain/suspension wise.
Based on what you posted in your original post, I would also recommend looking closely at Newells. We have some friends that have had several and they are extremely nice rigs with a long history of lasting over time as well as brand loyalty.
Mike.
