I have a 2004 38 ft Dutchstar, Spartan chassis with the same Cummins 370 BHp engine and 145,000+ miles. We have had it since 2014. We live in Colorado and it has seen plenty of Rocky Mountain passes pulling a toad. Engine has great torque and never had any power issues and has always run cool. Very few maintenance problems, mostly fluid and filter changes. Late last year had to replace the fuel pump and there was a technical bulletin to replace the pressed steel oil pan with a cast steel oil pan to correct oil pan gasket leaks. Cost was $6000 plus.
Something to remember -- A semi driving friend once told me you can go uphill too fast many times but you will only go downhill too fast once. He suggested using the same low gear you used going uphill to go down the hill on the back side and to use the transmission/exhaust brake and service brake to keep the speed down.
2004 DutchStar DSDP3812, Cummins ISL 370Hp, Spartan MM Chassis, 2017 Chev Equinox toad, Blue Ox Aladdin Tow Bar, FMCA #F451747