yellowstone grand tetons without reservations

We have really enjoyed our stays at KOA West Yellowstone. In Jackson we stay have stayed twice at The Virginian, which is a gravel full hookup lot in back of the small Virginian Hotel. Very central location, not much of a park, but clean, friendly, and access to the hotel pool. Often when we make our plans we can not be sure of the exact date we will arrive at the end point destination. Many parks will permit you to make your reservation in advance and then adjust the arrival date a few days before arrival or even the day of arrival. We have called campgrounds on many occasions over the last 40 years requesting a change in reservation for a day late or a day early. I have never been rejected. Perhaps we would have to switch spaces to accommodate other reservations, but never a rejection. Now this has always been at private campgrounds, KOA's family run campgrounds, etc. I have no idea what the rules might be at State or National campgrounds. I will add, that if you have any RV problems in Yellowstone or need to rent a car of van, contact Big Sky Rental in West Yellowstone. They really went out of their way to accommodate use when we had RV problems, so I recommend them every chance I get.
Do enjoy, and Happy Trails.