The Colorado Rockies are a very popular place to camp. However, with very few exceptions, I had no problems getting reservations this year for the 10 (so far) trips that I already have scheduled. It's a matter of knowing when the reservation windows open, and the best time to be on line to make a reservation. Having the website up, the facility map open, and your credit card at the ready, makes a difference.
Yes, I'm competing with all the other weekend warriors out there, leaving on a Friday and returning on Sunday. Since I'm semi-retired and work part time, I'm somewhat at the mercy of my work schedule. But prior to retirement working full time, I worked a 4-10 schedule with Sunday/Monday/Tuesday off during the summer. Best schedule ever! I could go up when everyone else was coming down, and have most campgrounds to myself, with very few other campers, on Monday and Tuesday. One of the perks for that full time job! When I retire again for good, I'll do the same thing.