Becker---you are forgetting this motor runs all the time the truck is working, not just mileage. If your fuel usage is 1/2 for travel and 1/2 for cleaning then this would pay off in less than 75,000 miles by your calculation---
Motor life will at least double compared to gasoline usage.
Ron---CNG, Propane, LNG are all different. CNG has more energy than gasoline not like propane and LNG which have less.
The computer used to run the motor in this van is completely designed by GM to work on both CNG and Gasoline. You do not have to aftermarket adapt this to work on CNG. This appears to be an aftermarket conversion so it will run on both gas and CNG.