We only made reservations in Denali and over 4th of July weekend. If we knew our destination the day before we might call for a campsite then, or while we were on the road.
I second the Tour Saver Book.
When we went in 2008 from Maryland, the Going to the Sun road was still closed when we got to that area in early June. We waited to do Glacier NP on the way home.
I don't know if you are planning on going to Denali, but if you do take the park camper shuttle bus (green bus) into Wonder Lake. Do not take the tan bus tour. The camper bus is cheaper, and the driver will stop for animals for photos the same as the more expensive tour bus. You will need to provide your own lunch, however.
Our 2008 Alaska trip blog link is in my signature.