Hi Sheri,
I did this trip last September. I notice that this year the trip is the reverse to what we did. We started in Rapid City and ended in Las Vegas.
The scenery on this trip is fantastic. I especially liked Bryce canyon, the colours were amazing.Yellowstone was marvellous, the only disappointment was not seeing a bear but we saw deer, elk, moose and lots of bison. The mud volcanoes and geysers were interesting too.
The trip itself is quite hectic with a few early starts and a couple of long days but overall it was a memorable holiday. We had a great bunch of people and everyone really enjoyed it.
As I recall, we didn't have any, or hardly any, rain at all and the temperature from the start was in the 70s and it increased as time went by. When we reached Las Vegas it was very hot around the mid to high 90s.
Depending upon the time of year you are going, you probably won't need any heavy clothing but as you head northwards you will find it getting a little cooler in the evenings so you might need a sweater or light jacket.
Regds
Jim