I have no idea of the cause of failures of those Motor home diesel engines without looking at what was done.
I've seen of a few, later models having failures due to heat from the cats now required and the new emissions equipment.
You can make a diesel fail by increasing times and fuel injection pressures etc. It increase power and fuel mileage but you can reach a point where the piston heads melt and ring lands. That is an unusual circumstance.
Perhaps their failures were simply algae in fuel and clogged up lines etc.
But generally a diesel's first major engine service was at one million miles.
Pumps, turbos, injectors and fans may go bad as with any machine but the engine itself is pretty stout.
May be they did drive it and let it sit for long periods of time without changing the oil for say five years and the oil level got too low.
If you buy a used motor home ask for/demand a warranty on the engine for five years. It may already covered under the manufacturers warranty, for a few more years, ask.