That sounds like a wonderful trip; I'm envious too. (-:
By late September the weather may already be pretty chilly, so be sure to take some warm clothes.
In the summer I would have suggested seeing Glacier National Park (US) as well, since it's on your route, but by that time of year some sections of the road through the park may be closed, as it goes over a high pass and they evidently do a lot of road work in September, after the summer peak season but before the snow (this may be true in Canada as well). So you might have to go in and out on one side, and there might be delays.http://www.nps.gov/glac/planyourvisit/gttsrfaq.htm
And remember that the whole time you're in the Rockies, the roads will be steeper and curvier, so you won't be averaging 70 miles an hour by any means.
If you're planning to visit Yellowstone, you might need to make a reservation ahead of time, even after the peak summer season. RV sites seem to be at something of a premium, and some of the campgrounds apparently start closing by September:http://www.nps.gov/yell/planyourvisit/camping...
There are probably also RV sites in the little towns just outside the park, but that atmosphere may be more commercial. You might be able to e-mail the park campgrounds and ask if you will have trouble getting a reservation at that time of year if you wait and call a few days before you get there, once you see what kind of time you're making. It might depend partly on the size of your RV.
If you do get that far, I would be sure to take one additional day if you can, and go on to Grand Teton National Park, which is also beautiful and typically not as crowded.
I'm not sure whether any of the US parks have been affected by the forest fires, which have been unusually bad in many places in the western US this year. I think the worst ones have been in Colorado and northern California, but I noticed that the Yellowstone website has some warnings up about fire dangers and camping restrictions, so you might want to take that into account as well and keep checking back.