Hello and welcome to the forums.
Airstream and GM/Isuzu have shared a long relationship with each other. I'm not 100% sure when the very first Isuzu 6BD1A with BAE aftermarket turbo system was installed in an Airstream MH but I have seen a 79 with a turbo diesel.
The Isuzu turbo diesel MH's were 28',30', my 310 (quite a few more of these), 325's and 345's. The 35, 36 and 37' Classics were all Cummins motors I do believe. There was a model change in 84-to 85 with the 325's I do believe, where Airstream used the 6BD1T motor with it's better oil delivery, heavier pistons and crank. However the little more hp that the motor produces is more than soaked up by the heavier coach with its tag axle, longer length and heavier weight.
The Isuzu is a quality piece of work with its ability to cold start without the use of glowplugs right down to 10C. Parts can be hard to come by but since the motor was used in ships (generators and power plants), excavators and other heavy equipment, you can still get rebuild kits. There are a few differences between an excavator diesel and ours, mostly fuel governor and fuel delivery as these motors are meant to run at a constant rpm.
As stated before, buying an Airstream with a tag axle can present problems as servicing and buying parts for the tag and its braking system are almost non-existent.
A few people have transplanted newer 12V
Cummins motors and 7.3's into older 454 Classics but they're rare and I would be wary. Someone has even transplanted a new modern Duramax diesel but last I heard was still having issues with PCM's.
The cost to convert to the diesel in 1983 dollars was $15,000.00 US, as all P30 chassis were delivered to Airstream as 454 models, so Airstream did only a few as compared to the gasser. Thusly it can be difficult to find them, but they are out there, and if you do find one, be prepared to pay a premium over the equivalent gas model. The 95-96 36' is the pinnacle with its single axle and better transmission BUT you will pay a huge premium to buy one of these coaches. I have seen a pristine version with an asking price of $125,000.00.
My 310 serves me very well with the amount of room and ease of service. Good luck with your search.
PS Since buying mine I have been noticing prices for Classics are on the rise, especially for the diesels.