Jon, in some cases that is more true. Many of these engines can have the EGR removed and blocked, DPF removed and trashed and you may have to reprogram the ecm. I have been there and done that on trucks and wouldn't be afraid of doing it on a coach. Emissions standards are set by the vehicle year in most states. There are some advantages to owning emission exempt aged vehicles, this is one of them. Now I'm not advocating anything, just saying!
Note: Most of these engines are showing standard ratings and can be turned up. Like I was reading recently on the Cummins ISL 8.9L in motorhomes and buses, it can be cranked up a bit, but most are set at a comfortable safe hp/torque level for the Allison 3000 transmission. Of course there are some fuel incentives in this scenario as well.
We are currently eyeballing one on this list for what may be a near future project in an older classic Newell.