Cummins New 5.0 V-8 Diesel?

I have a 1995 Dodge diesel 12 valve on a 0ne ton extended frame. Its a Scotty motor home. I get 14mpg running 65 to 70 on the highway. I don't know why the Cummins is not use in class C motor Homes. I have 61,000 miles on it with no trouble at all. There nothing out there to match it. When I pull the car trailer with a 40 Ford in the trailer the mileage does drop to 12mpg. I just replace the front brakes and exhaust. I would get a new Class C if it had a Cummins. But first I want to wear this one out. It only 20 years old and still looks new. We only put about 3,000 miles a year on it. I know I don't run as much as some of you. I still work taking care of my rentals and trailer park. I did buy it used paid 21,000 for it and it had 13,000 on it. It been garage kept since new. I change oil every 3,000 miles on it. It small enough that I can park about anywhere. Also my wife will drive it.