A rule of thumb for motorhome power is that you need about 1 hp per 100 lbs of actual weight, coach plus anything towed. That gives barely adequate power, in my opinion, so yes you would sometimes be slogging up the steeper grades (4% or more) in the right hand lane cause you don't have enough reserve power to pass. Rigs with that power ratio do well enough with a running start at the steeper grades, but once they get bogged down in traffic they probably cannot build up speed again. So, if that 45,000 GVWR coach weighs near that much when loaded, and you add a 5000 lb toad, you are probably going to be disappointed in the performance. Your remarks suggest you would find that frustrating, so you probably do want the 600 hp engine (but be prepared to pay for it in fuel economy).
Even with 600 hp, if you are expecting performance similar to your car or light truck, it ain't gonna happen. A typical car has a hp/weight ratio down around 1:15 and light trucks in the 1:22 range. In other words, 4x or more power than any motorhome.
Kevin gave you realistic expectations for hill climb speeds. My own coach, powered with the 370 hp version of the same engine, weighed 37,000 lbs with the toad and gave similar hill-climbing speeds. You get used to it after awhile.