Welcome to the forums!
Not to parrot a famous song, but this could be heaven, or this could be hell. Either way, it'll be an adventure. I cannot claim great expertise either with renting an RV or Arizona at that time of year, but I have done my share of camping and have been in those areas, though not at that time of year. The number one suggestion I'd offer you is to take the adventure slowly, maybe do Sedona and either Flagstaff or the South Rim and leave the other places for another time. The whirlwind tour approach whether by car or RV leaves most of your time in the vehicle driving from one place to another. The great beauty of FV travel lies in being there, rather than getting there. There's enough to do and see in both Sedona and the Grand Canyon to take a week in each. But that's just my humble opinion. Whatever you wind up doing, savor it, make great memories, have fun, and best of luck getting the stars to all align.