Three trips in the last five years and the only reservation that we made prior to leaving home was in Seward for the week of July 4th. As our time neared to be in Denali, we made reservations a couple of days in advance. The first time we were able to get Denali the day prior to our arrival.
Depending on when you would arrive, I recommend making reservations for Valdez once you are within a couple of days of arrival.
Otherwise, we stayed in provincial or state parks or we did quite a bit of boondocking, especially in the Kenai/Soldatna area. We were at Fred Meyers in Soldatna and Walmart in Kenai. Of course, we joined the boondocking crowd at the Walmart in Whitehorse.
Just take your time and stop when you want. Reservations just cut into the freedom of being in BC, Yukon and Alaska.