I went back to your first post, which shows your bus as a 1990 Chevy. Helpful tip, you may want to go into your profile to list your bus info there, so that it will show up on your posts. That way, you won't get your questions answered with that question.
This connector looked familiar to me, and I thought it looked a lot like a connector for most GM vehicles I've seen from the 80s / 90s so your bus being a 1990 GM fits. The C-series medium duty trucks were likely beginning to offer cruise control around that time, and it was quite common in that era for vehicles to have harness connections for certain options whether they were to be equipped with them or not.
Even if it's not for cruise control, it is likely there to connect something the bus was not equipped with, like a sensor that wasn't necessary for how your bus was equipped. Cruise control or governor would be my guess. Or it could be a block connector for sensors and distributor for a gas engine. Definitely looks like a connector GM would use from the factory on most vehicles of the time. I'd recognize those keyed circular pin shrouds anywhere. Mostly I saw them on engine sensors.
The 8.2, by the way, from my understanding, isn't a terrible engine, but if memory serves, its Achilles Heel has mostly to do with injectors, and the 'Fuel Pincher' setup makes the exhaust reek. Biggest hurdle is finding a diesel mechanic that actually understands them and doesn't mind working on them. Finding parts may be an issue in the years to come, if it isn't already. The upshot is, with a purely mechanical setup, if you wanted to swap it for something else, factory mechanical injection makes it easier than if it had, say, a DuraMax in it.