We also are spending a month in Colorado on our way to and from Utah this fall. I have reservations at Moraine Park Campground in RMNP, Cheyenne Mountain SP in Colorado Springs, a commercial Campground in Buena Vista, West Fork NFS Campground outside Pagosa Springs, and Haviland Lake north of Durango.
When I make reservations at the first possible times in CO state Parks and National Parks we are finding very limited site availability.
Thanks to 4Runnerguy for some great recommendations. I recommend anyone coming to Colorado review his archived posts.