Sounds like a great trip - the Aspens will be turning then. On your way from Mesa Verde to the Sand Dunes, you will pass through Pagosa Springs. You should stop there for lunch at least - there are some interesting things to see. Also, when you get to South Fork, you might want to take a side trip up to Creede, which is an old mining town that's pretty lively, at least in the summer. The headwaters of the Rio Grand is just west of Creede, at the Rio Grand Reservoir. I know there are many National Forest campgrounds there but they may be closed by then. However, there are lots of RV parks in South Fork because it is a very popular summer destination for part-timers. We spend many weeks every year in Colorado dry camping and riding our Polaris RZR.