If you were to try those same kind of grades by not manually shifting gears at all, but instead by just pushing the button on the gearshift lever to put it's five-speed transmission into Tow/Haul mode, then pushing the gas pedal down and letting the transmission do it's thing .... you would find that you can go up those grades like you want (55-60MPH?). However when doing this you will notice that the tachometer probably shows RPM in the 4500-5000 range and that the high pitched engine sound makes you uncomfortable.
The V10 is, for a truck engine, a relatively high RPM engine. You have to rev it to tap it's horsepower and torque capabilities. If you're maintaining the V10 as you should, using it up grades in a high RPM mode will not harm it. You just have to get used to driving you motorhome this way up grades.