If you're retired, don't go to Yellowstone in the summer. You'll enjoy it more in June or from the last two weeks of August thru Sept. Same for Grand Teton Nat'l Park. Fall is a beautiful time to be there. In June you'll see some baby critters.
Yellowstone has many parks and quite a few are not even reservable. Stay outside the park the previous night and drive to a campground around 9-10am when folks are leaving and you won't need reservations. We didn't with our 40' motorhome. Even at the campgrounds that accept reservations, there are always cancellations first thing in the morning.
Here is Yellowstone's site. Every national park has an excellent site so check them out for things to do, campgrounds, road issues, etc.
YellowstoneFor Grand Teton there are two campgrounds (dry camping but water fill, dump station and showers) that don't accept reservations. Each one has 300 sites and rarely fill - even in summer. We like Gros Ventre campground. The other is Colter Bay campground. There's also Colter Bay RV with hookups which is right next to the campground.