Since you are retired, you have options that working folks and those with school age children don't have. Namely, to avoid the months of July and August and travel during the "shoulder" seasons of Spring and Fall, when many non-retired folks and those with children are stuck at home.
For example, we left NJ back on May 19th, arrived at Grand Teton NP on May 30th without anything but same day reservations along the way (including over the Memorial Day weekend). We then spent the first two weeks of June in Yellowstone and have been back at Grand Teton since the 16th. We do mostly first come-first served campgrounds and only go to a FHU campground when we need to dump tanks, about once a week. Not needing hookups all the time greatly increases our campground options. During our month at Yellowstone/Grand Teton, we made several same day reservations and a couple 3-5 days ahead.
It's getting crowded here now, so time to head for home for the Summer.
