My "old Crown" came with the Detroit 6-71 equipped with the turbo. It has always run smooth and got 10 mpg with a top speed of about 65 mph on a good day. On interstate 5 just going over an overpass could drop the speed by up to 10 mph. The grade going North out of Dunsmuir meant going 18 mph in 1st gear and being passed by heavily loaded 18 wheelers.
The "new Crown" (on its only trip up I5, to bring it home) was going 54 mph at the bottom of the grade and 62 at the top of the grade at Dunsmuir. I got about 6 mpg on that trip. The new Crown easily went 70 mph on I5 ( past redline but I didn't know it cause the tach quit working)..
For me my skoolie is a vehicle that I can stay in comfortably while I go traveling. As the primary purpose is a vehicle vs. a home, drivability is important. Part of drivability (to me) is enough power to make the hills and cruise at highway speed getting to the next destination or waypoint. To me both the Detroit and the Cummins sound cool. Based on experience the Detroit was easier to start in real cold weather. I never had a major problem with the Detroit, while I had to replace the Cummins already.
For the moment the scales tip in favor for the Cummins for the sufficient power and maintainability (after all they used them in 18 wheelers).
I won't be driving all the time, or staying in the Crown except when traveling. It is each to their own life style. For those who wish to stay in their skoolie, its all good also. I don't have any negatives to say about anyone else's skoolie as I have not driven or worked on anything but the Crowns.
The beauty of the whole skoolie experience is creating something useful and practical and making use of it when done. Back in the 70s I built three different Harleys and enjoyed riding them when done. I still have the third one (I built from scratch). In some ways those who build skoolies are like those who build custom bikes, we create and make use of our creations in a practical way.
Thanks for reading.