We had the good fortune to stay at the Lakeside KOA during our recent trip to Fort Collins. The sites are spacious, and the campground is run well.
Talk about luck! We had one of two RV sites up near the lake. They were incredibly roomy, level, and shaded! There is a fantastic walking trail around the lake and saw a good number of fellow campers fishing off of the pier.
The staff was very friendly and attentive, and the restrooms were clean and private. The only issue that we ran into was the Cottonwood trees were blowing their seeds when we were there (second week in June). Definitely felt like it was snowing but having the trees and shade was well-worth the annoyance of shaking off the cotton seeds.
We highly recommend this well-run and spacious KOA if visiting the Fort Collins area. Oh... as of June 2021, you do need to have reservations to enter Rocky Mountain National Park. Highly recommend booking well in advance, but if you forget or didn't know, the NPS does release 25% (or 33%... can't remember exactly) of the next day's passes the night before precisely at 5pm. We had three of us online at 5pm, and all three of us were able to access passes (only one of us actually got the passes for the group). By 5:03pm though, they were all gone.