It has been 9 years, since we traveled to Alaska; may the situation has changed.
We made two reservations, before we left home. We left home at the end of may and the reservations were in July. One was in Anchorage - the granddaughter of our traveling companions was flying in to spend a week with us. The second was in Homer - we had booked a fishing trip from there.
We took a month to reach Alaska, enjoying the trip. Since our phones did not work in Canada, we showed up at campgrounds to see if they had two sites available. Only once were we turned down. We found another campground in the same town. That was in Dawson Creek, the start of the AlCan Highway.
Other than what I mentioned above, we had no set itinerary. While in Alaska, where our phones did work, we would decide on our next destination and call ahead to make reservations at our next stop.
Have a great trip. We're planning to go back next year.
Tom