Different view here: We've traveled to all those areas in September/October and to us it is the best time of year. Less crowded, beautiful skies/leaf shows, etc.
Sure, some places will be closing up, particularly in Canada and Yellowstone area, but plenty will still be open. And though you can get snow in those areas, it's not common, and all you do is wait it out. Northwest US will be fine in September.
Years ago we Rental-RV'd in the Southwest with a 6 week old, and then again with twin 6-month-olds. As long as you are equipped, you all will be just fine, especially if you use the built-in food supply.
I recommend you at least check on campgrounds along your route to be sure there are some open. You will not need reservations at that time of year except on Labor Day weekend.
