I've never owned one but looked into it at one point. Those 6.5 diesels are not know for power. They were built for good fuel mileage which may or may not have been achieved. I think they were around 180 HP. A 454 of the same vintage has much more horsepower and about the same torque. The 6.5 came in turbo and non-turbo versions in about those years. I would definitely get the turbo version. The automatic trans. should be a 4L80 I believe. They're a decent unit but lack the tow/haul and grade brake features of current autos.
Do you want to know about the Trek or the Chevy diesel?
We had the diesels in Suburbans we used as our mailcars on 400 to 600 mile per day runs, six days a week (there were three routes) for 120,000 to 180,000 miles a year. These got oil changes at the weekly inspection on the down day, and other routine maintenance per GM's schedule, or what came up on inspection. The runs were mostly steady speeds on rural highways. I think most of the Suburbans were kept in this service six to eight years before replacing them, and don't know that any of them were ever overhauled, but the transmissions got rebuilt every 2-3 years.
We also had stretch Suburbans we used as airport limos, fleet of three making 10 to 18 100 mile round trips daily, so an average of 500-600 miles a day with maybe 20-30 minutes sitting idling per trip. These got weekly service on Saturdays and Sundays when the trip schedule was abbreviated, one or two could handle the runs and one could be out of service all weekend if necessary.
These were 454 gassers. They got engine and transmission overhauls every 250,000 to 400,000 miles, stayed in service that way at least 12 years, until a combination of increasing traffic and safety standards pushed us to motorcoaches.
I don't think it is likely that you will push a GM chassis motorhome, gas or diesel, past the engine service life. But I don't know how the diesel handles sitting most of time in storage, running only occasionally, as that is not how we used them.