IMHO I would add 17 miles to the trip and change your order slightly. Red Lodge to Devils Tower to Rapid City to Flaming Gorge.
We stayed in Deadwood SD at Whistlers Gulch after leaving Devils Tower and when we left Deadwood we visited Mount Rushmore, Crazy Horse and The Mammoth Site in Hot Springs. Highly recommend adding the The Mammoth Site to your stops. Then spent the night at a very nice RV Park in Lusk WY called BJ's Campground. Did all three and the drive of 135 miles was in one day
We went from Lusk to Estes Park then to a fantastic park Rifle Falls State Park NOT Rifle State Park but Rifle Falls, and then The Black Canyon of The Gunnison. Was not impressed with Estes Park but the others WOW.
