My experience with the old 2-stroke Detroit Diesels has been that they are rather limited in current usage. There is an extremely small minority of the RV community called "schoolies" who renovate school buses into motorhomes. Some of them are professionally done, however the majority are cobbled together. I rarely see any of them at any RV parks, possibly because they are limited to 55mph due to gearing.

There is also a small minority of professionally-built bus motorhome conversions that are still on the road, although I rarely see them either.

Here's one couple that shares their story of a professional bus conversion and later the rebuild of their 2-stroke Detroit engine:

Technomadia's Home: GM PD 4106 Vintage Bus Conversion | Technomadia

I got to ride on a bus in Canada for whitewater river rafting that had a Detroit engine.

Farmers use them due to their massive amounts of torque and they could be economical for very short hauls.

Church groups use them for field trips and many of them end up in Mexico to be used for commercial transport.

I really don't see any of these groups using them enough to cause any issues with air pollution. This is simply Agenda 21. I think in real life they will be coming down on the little guy who has been using the same truck for a long time, while favoring big business that can afford to buy new DEF diesel technology and rack up enough miles to justify the cost.

Dkr.