Steve, in answer to your thoughts here are some facts I uncovered a few years ago.
The early diesels were underpowered. They had a turbo that was after market, (if they had a turbo at all), and less efficient transmissions. Airstream recognized that and in 1984 started to correct the problem. The later models were far more powerful and efficient. My rig is rated at just 220 to 230 HP.
If you want to install a diesel in your rig, just be sure it is large enough. Efficiency is an outcome of proper sizing. Too small or too large produce poor economy, and too small produces hateful words!
In 1984-85 and later they got a whole new engineered engine with a factory turbo, cylinder oiling, heavier crankshaft, stronger connecting rods,and stronger pistons. In addition the cylinder head had improved valves. You can verify this information directly with Isuzu's Industrial Division. The improved engine had 30 more horsepower than earlier models.