The Cummins ISM is (was, actually) a 10.8 liter engine, typically produced in the 500 HP version for motorhomes with 1,550 lb-ft of torque. The ISL is an 8.9 liter engine, produced with horsepower ranging between 350 and 450 HP, with a ton of 400 HP versions being made. The ISL has 1,.200 lb-ft of torque, although the latest years now have 1,250.
Both are good engines. The ISL is very popular and about as bulletproof as it gets. The ISM was discontinued when the EPA 2007 emissions came in and replaced by the ISX12 engine. Obviously, the extra torque and power was welcomed, especially on the heavier 45' coaches.
The only downside I've heard of on the ISM is that they use a dry PTO shaft. This shaft drives the air compressor and isn't lubricated by the engine oil, like the other engines are. That's because it's not at the front of the engine in the timing cover where it gets splash lubrication from the engine oil. Instead it's a rear takeoff back by the bell housing so it has sealed (aka - dry) bearings. That's fine until they fail after a number of years and miles. When that happens it's a real bear (labor intensive) to get to them to replace the PTO shaft. Not so bad on a line truck but extremely difficult on a motorhome due to the bedroom floor. The newer ISX12 didn't have that problem.
Mark & Leann Quasius
2016 Cornerstone 45A
2007 Allegro Bus 42QRP (Sold)
2012 Jeep Wrangler Unlimited - Rubicon