Soritng out drive trains

At that age I'm much more concerned with how it's been maintained than powertrain specs.

x2. Running properly there few places you won’t be able to travel in any DP.

Yeah, you're probably focusing on the minutia instead of the big picture. Miles, proper maintenance, and floor plan.

almost all will be turbo, and with an allison transmission
the engines will Caterpillar and Cumins , with a few others in the mix

slides versus no slides, roof leaks, water damage, fiberglass body De-lamination
bus entry/mid-entry

The thing to keep in mind about motorhomes is very few ever get enough miles on them to reach the point where engines wear out. Instead failures are generally either due to lack of maintenance, or just bad luck, with a few exceptions. For all practical purposes any drivetrain combination will work, though some of them will be slower about getting you there than others, particularly if you travel in the western mountain states.

So as already mentioned maintenance should be one of your key concerns, the big issues tend to be leaks in the roof of an RV, and wear/aging of mechanical components, the big thing here is anything made out of rubber will tend to dry out, crack and fail somewhere in the 10=15 year age range. This means, belts, hoses, rubber bushings, suspension air bags, etc. This can also include parts in pumps, pressure regulators, etc.

Keep in mind that maintenance on a DP is not trivial. It can easily cost $6k to replace tires, a couple hundred for an oil change, etc.

I have a unpopular Detroit series 40 that I purchased without a single service record.
So far so good.

Late 90s and early 2k there were a few more DP chassis choices than today. For one, there was the Magnum chassis under the Safari, Beaver and Harney coaches. These were manufactured by SMC in Oregon and were mostly CAT 6 cyl/Allison 6 speed drive trains. I wondered when I bought my Sahara if I'd be able to get chassis parts, but so far I've had no issues. I do have a problem getting parts for appliances from manufacturers that are still in business though :(

Also consider the suspension on the chassis. Air bags provide a smooth ride but contain more parts that break. The Magnum chassis had a Torsilastic suspension called Velvet Ride that people tended to hate. I have the leaf spring/shock suspension and it's harder than air bags but doesn't bottom out, which the Velvet Ride can if not adjusted correctly.

Although CAT no longer sells engines to the over the road market, there are CAT service shops everywhere and the engines are pretty much bullet proof if you do the normal amount of yearly PM (fluids and filters). A call to CAT's hotline (877-777-3126) with the engine ID will get you all the service done on that engine over it's life.