The Cummins ISC is arguable the best MH engine available, although the ISM (perhaps even better) is available in smaller numbers. The ISC has had a few cam related problems and exhaust header issues. The header issue is easily fixed. The cam issue is relatively rare but usually catastrophic.
In our MH we get 8.3 mpg at 33k, towing ~7k we get 7.3 mpg. I've towed from coast to coast, sea level up to 11,300'. Non towing is will pull a 5% grade at 55-60mph. Towing at 11,000' at 9% was 28mph.
Jim and Jennie, Cats=Bittles and Potter, 2000 Dynasty 350 ISC
2013 Silverado 4x4 Towed with R1200GS in bed.
PROV23:4 Do not wear yourself out to get rich; have the wisdom to show restraint.