Here in west central Illinois, we've been running in the 80's and lower 90's and often with high humidity so heat index 90's to 100 or more. Occasional breaks in the 70's. That's typical and will be through August. Adjust a little up or down south to north. Another factor, if you wait until at least some schools have started (mid to late Aug) the typical campground population will change from families and young to seniors; quieter, less crowded, earlier to bed, etc. You didn't say what you mean by the "circle" or if you will start across the north or south; big difference. May through June would be good unless you make it to high altitude(cold).