Drive it before you condemn the HP rating, the L10 Cummins 8.3 engines are used in a lot heavier rigs than a class A motorhome.
HP isn't everything, personally I don't pay attention to HP ratings but I always look at TQ ratings, My last class 8 truck only had 500 HP Cummins...... but it had 1850 ft lbs of TQ . Torque is what you need to get moving.
My brothers 35' Daemon Class A diesel pusher has a 275 HP 860 TQ 7.2 liter CAT 3126 in it and it flat gets down the road. Gets 10 mpg running about 70 - 75mph.
Accelerates very well and will run over 90mph all day...... He got a high speed driving award in Michigan this summer to prove it