Colorado in September??

We are headed to Colorado the end of this month for a long trip. What surprised me the most was finding places to stay as most all RV parks were booked solid with a date here and there open, but no 3 - 6 day straight stays available. We ended up finding a NF CG that had 2 sites left and snapped up one of those. Yes, good time to visit, but everyone seems to have figured this out too. Best bet may be trying USFS campgrounds and hope for the best.