Two years ago, I spent a delightful month of September on Lake Powell. Congratulations ! The water will be perfect for long swims off your swim platform. The nights will be cool, the days hot. Can't imagine a better spot for an R-25 in September.
Two (2) words of caution, before you depart:
1) Diesel is scarce throughout most of the Lake. As I recall, Antelope and Wahweap Marinas at the South end, and Bullfrog at the far North end, have diesel available. All the other Marinas, or fuel docks, on the Lake only have gasoline. So, you have to plan your trip(s) to have reserve fuel to return to one of those points, or else !!
2) When pumping out your waste tank, you will find the dock pump hoses have a fitting for houseboats, their main customers. It won't fit your R-25. You can buy a cheap adapter, I believe, right at the Store on the Wahweap Marina. Do so, before your cast off the first or second time. No adapters available up-lake.
Have a great September, Charles
p.s. If you click on my photo album, it's all Lake Powell, to whet your appetite further.