We did not have any problems getting a campsite on our last trip through the Rockies. Sometimes dump station and water only at the entrance and 15 amp electric. The only time we couldn't get a campsite was in Jasper NP on the July 1/4 holiday weekend, however Parks Canada did open up a huge overflow site, capable of holding hundreds of RVs. No hookups, but that's why we have an RV.
In short, 6 weeks from June to August, no problems getting a site, sometimes not ideal (for some) but always had a safe place to spend a night.
Ken