Why not just go and not make reservations? You won't have problems at that time of year. Then you can monitor the weather as you get nearer to those parks. It makes for a more carefree vacation if you don't make commitments with reservations. :) We rarely made reservations during our 16 years of full-time travels.
I don't think you'll have problems with snow in September and no, you don't want to have to worry about chains on a RV. If that would happen to occur - which I really don't think it would - you just spend an extra night in a campground until the roads are clear. In Sept. it's not going to stick and build up.
Go and have a great trip!
