As you describe it, your rig has normal power and handling for a stock class C 30 footer. Put 65 psi in fronts and 75 psi in rear tires. Sounds like this is the first class C you have driven. It is not surprising that you are going 40-45mph pulling a toad up a grade. Use the lower gears for climbing and to save brakes from overheating on downgrades and accept slower speeds for driving in hilly country. Class C's are heavy and top heavy and have slab sides. Big rigs passing on the left will suck your rig to the left, you'll develop reflexes to control that. Take frequent rest breaks, get feet on solid ground for 10 minutes. Try to enjoy the scenery at 55 to 60mph instead of trying to keep up with car and truck traffic, and do 500 miles per day. Don't let yokels push you to take curves too fast for safety. Make lane changes with great care to avoid side swiping cars and motorcycles in your blind spots. Interstates often have cupped concrete sections which give a rough ride. Secondary blacktop roads are often smoother and scenery is better.
After you get familiar with suspension and steering upgrades, and talk to other class C owners about their efficacy, decide on what you need. Claims of Class C's handling like SUV's are usually exaggerated.