Last summer, we got our reservations for the Riley Creek campground at Denali three days before we arrived. That was the farthest out that we had any reservations on the entire trip.
Last July, we made reservations for the Skagway to Haines AMHS ferry one morning and were on it at 3:00 that afternoon. We made reservations for the Kenai Fjords tour boat the evening we arrived in Seward and were on it the following morning. And, on other trips, we have just walked up to the office and gotten tickets for flightseeing around Mt. McKinley from Talkeetna, the narrow-gauge train in Skagway, and the Alaska Railway from Anchorage to Denali.
As Sue said, Alaskans recognize the difficulties of travel in their state and make allowances and "walk-ins" are welcomed everywhere. After all, many of them have to make a year's wages in 5 months!
About the only exception we ran into is Skagway, where everything is designed for the cruise-ship crowd and scheduled months in advance. But Skagway doesn't care about RVers anyway, as shown by the plan to close Garden City RV Park. So, after a couple of trips there, it has become primarily just a place to catch the ferry to Haines or Juneau for us.