I run the 8.3L mechanical 300 hp Cummins on my 31,000 lb rig. It will do 6% grades at 35mph with or without a 4,500lb toad. Torque is about 860 ft lbs.
I believe the rig you are evaluating will have over 1,000 lbs of torque which should be adequate for a single axle coach.
Speed uphill is overrated since you will not spend much time in that attitude anyway. It is just an ego thing.
We spend most of our time (@50,000 miles) in the Pacific Region and its mountain ranges with not a single failure of climbing the hills. We just do it slower than some trucks and faster than others.