I would also vote for US 50. As suggested, spend the first night after Grand Junction at the Black Canyon NP campground. Cerro Summit is the long climb on US 50 between the Black Canyon and Blue Mesa Reservoir, which generally has some construction because it is on shale that is prone to landslides. After that you have a pretty level route until you reach the west approach to Monarch Pass at Sargents.
Plan to spend the second night on the east side of the pass at Salida. You should get there relatively early, so park your coach at Heart of the Rockies RV Park just east of town on US 50. Then tour historic downtown Salida. There are some very good restaurants, as well as businesses built around rafters and kayakers on the Arkansas River. The most popular rafting destination in the state is just north of town in Brown's Canyon. If you have a few extra hours, you might even take a short raft trip.
Before leaving Salida the next morning, take a look to the north at the Collegiate Range shining in the early morning light. It is one of Colorado's tallest and most impressive mountain ranges. Then head down the Arkansas River canyon to Canon City and Pueblo.
A few other attractions to visit if you have the time and interest. When leaving GJ, drive through Unaweap Canyon (the ancient valley of the Gunnison River) along CO 141 to the wide spot in the road called Gateway. The founder of the Discovery Channel has a ranch there and has built a unique automobile museum (see "Gateway Museum" on-line). Then in Montrose, check out the Colorado Historical Society's Ute Indian Museum before heading east to the Black Canyon. When you pass through Gunnison, stop at McDonald's for ice cream. Across the highway at the fairgrounds is one of the Denver & Rio Grande RR's narrow-gauge trains that ran through the Black Canyon from the 1880's to the 1950's. If you had more than two days for the GJ-Pueblo trip, I could recommend a lot more sights and side-trips because I have lived, worked, and played all over the state for over 50 years.