The construction techniques of a Prevost have been described already, which answers the OP question.
Unlike a coach built "on" a Spartan, Roadmaster, Gillig, or any other rolling chassis, a Prevost chassis includes the entire shell. Those coaches built "on" a rolling chassis are just that. There is no way to achieve the engineering integrity of a Prevost with an built-on solution. Don't take this to say there is something wrong with the built-on coaches. How many of them are expected to run all day long almost every day. Look at used Entertainer coaches. In less than 10 years most have well over 500,000 miles and some over 750,000 miles.
Prevost, MCI, Van Hool and a few others are built in a similar fashion. Newell, too is built this way, although it's never been clear to me what the life expectancy of a Newell is. I would suspect far longer than most will ever drive one in a motorhome application.
As for slides, there are Prevost's with Prevost slides after about 2000, and others with other slides. I doubt there was ever a factory slide in an XL, the stainless riveted side coaches. Prevost now offers up to 4 slides in motorhome shells. Entertainers either have one or two slides. And yes, there have been Prevosts that couldn't move because of their slides. I've never heard of one that got misaligned, but motors and interlocks do fail. Until recent years, Prevost slides had locking pins and rarely, but occasionally, the pins will not fully seat. If this happens the transmission will not go into gear. If you know how, these safety interlocks can be overridden.
Where Prevost conversions differ the most is in how the non Prevost systems were engineered and installed by the converters. Some converters didn't go overboard in unnecessary automation and air driven systems. Others went nuts with these systems (air pocket doors, high pressure Headhunter toilets, air assisted awnings, and the list goes on).
Almost all Prevost conversions are totally electric. There is no propane and few systems are DC powered. As a result, most have 2 inverter/chargers, up to 8 8D house batteries, 2 engine alternators, and a host of other electrical systems.
Some Prevost conversions include Prevost's over-the-road air conditioning. This is a massive engine drive system putting out 80,000 BTUs. Right sized for an all window coach with 55+ passengers. Overkill for a MH.
All but one Prevost engine belt on my coach can be changed without tools in less than 5 minutes. Paying passengers don't want to be stranded on the side of the road.
The list of difference could go on. Hopefully, you get my point that it's a bit unfair to compare a Prevost conversion to any traditional coach.
99 Country Coach
Prevost XL 45 Conversion