As much as I like to promote the beauty of Colorado, by the time the Balloon Festival is over, most of the leaves will have changed and dropped. I think I'd head east to Arkansas and explore the Ouachita and Ozark National Forests. Some great state parks to camp in. The Talimena Scenic Drive, Crystal Bridges museum in Bentonville, Pea Ridge NMP, Eureka Springs, Lake Ouachita, Blanchard Springs Caverns are all worth some time. (Talimena and the road into Eureka Springs may not be the best choices for your rig, but drop it and drive in with your truck.) Then head north into Missouri and explore the Mark Twain NF, Branson, etc. The leaves will typically be coming into their peak about that time.