A lot of wrong information being passed about throw away engines. The C-7 and the older 3126-E engine can be rebuilt. If you do not believe me just look it up in the internet and you will find sleeves that can be installed on the C-7 and the 3126 Cat engines. I have the 3126-E Cat engine on my coach and I have 70K miles on it now and it just broke in at 60K where it runs so smooth that I love the engine. It gets constant 8-9 mpg.
Not sure about the small Cummins engine if you can apply sleeves on it, but I do know that on the 350 hp Cat engines they can install sleeves when ever it needs an overhaul.
When I bought this coach brand new the warranty on the 3126-E was unlimited mileage or five years. Cummins does not give that much warranty and like somebody mentioned they have had their share of problems. The ISL 400 had a problem with wrist pins that were not machined proper and they never recalled the engines back. What they done was install a light on the dash so that it would warn you when it was going to blow up, so that you could have enough time to turn the engine off. And the smaller Cummins engine the ISB had a weak spot on the block and would blow a hole on some of those blocks that were manufactured south of the border. I had that engine on a Dodge truck and I blew two engines before I traded it in. Cummins did make the engines good, but it was disappointing to say the least.
Big engines are installed on big heavy coaches and if you have any mechanical know how, you will understand that the heavier the coach weigh's the more fuel it takes to move that weight.
Have a friend that owns a Wanderlodge Bluebird weigh's over 40K lbs and he told me he only gets five miles per gallon on a two cycle detroit diesel. V configuration, not sure what those engines are called, but it was a V-8 two cycle and I believe the coach was a 1993.
2002 Gulf Stream Scenic Cruiser 38'
Neway Freightliner chassis, 2018 Buick Envision Ess.
3126-E Caterpillar 330 hp