question about horsepower

i've been looking at class A diesel motorhomes over 32', and noticed large differences in horsepower. many have the cummins 5.9, with various hp ratings from 190 to 230. i read elsewhere on this board that 300 hp would be the lowest you would want to go for a motorhome 36'+, but what about a 34', would 230 hp be underpowered? can a 190 hp cummins 5.9 be worked to increase the horsepower? and what would to ballpark cost for this increase be? thanks.