It does sound likely that yours has an aftermarket particulate filter added to the exhaust. I've never owned or cared for one but did notice on a trip to Cummins Rocky Mountain a while back that they do DPF cleaning. There's
a web page about their service; surely there are many other places that also do this.
For the short term perhaps you could locate the DPF and remove it..? That monitor would surely notice the missing filter, but whether it is installed in a way to impair/prevent bus operation when the filter is absent is anybody's guess. If there is a particulate filter in the exhaust and the monitor says it's time to service it's probably because the filter is getting too restrictive.
It's time to go poking around the engine. As I recall, my 8.3 has just the exhaust manifold, turbo, some pipe, and a muffler. Check whether yours has additional stuff in the exhaust path.