As our tag line shows, we have a 2006 Damon Astoria, 38', with the Cummins 300 hp ISB diesel. Yes, it is underpowered. No, we don't do much mountain driving, as our trips mostly (but not always) involved Florida and Disney. I recall most postings here recommending 10 hp per foot of length as being ideal.
Having said that, however, we can certainly pull our Ford Explorer toad or my 20 foot fiberglass heavy Fish/ski boat to Canada. And what I don't have in power I make up with in the better mileage I get with the 5.9 over the other engines. And the DW has no problems cruising with our RV at 70 mph in spite of my protestations to slow down....