If you are wanting to stay within the national park, it would be well to start working on the reservation system right now. You need to find out the "window" of dates when you can reserve a campsite and be ready to try to get that site immediately.
We have stayed in Estes Park many times but only once in the park. We stayed in the Glacier Basin campground. There were very few sites that would accommodate our 34' fifth wheel. We were very fortunate to find one that did--by working on it far ahead of time as above.
The other campground in the park is Moraine Park, but I do not have personal experience of that campground.