Avoiding the crowds in the summer also means not having some of the attractions open.
Do you have a destination in mind? When we plan, we go to the websites of the National Park or Visitors Bureau and read their recommendations.
Both May/June and Sept/Oct. have less crowds almost anywhere in the country. To avoid the snow, you're looking at July, August, September if you're going in mountain country. (depending on elevation, of course.)
2015 Dynamax REV 24TB class C