The only reservations we made for the whole summer was the July 4 weekend (Alaskans like to camp, too) and 5 nights at Denali's farthest campground you can RV - Teklanika. Those were only made a couple weeks away when we could better judge where we'd be. As it turned out we were a little early for Denali so on a whim we boondocked nearby at a lovely spot and pulled into Denali early morning when folks were leaving. We easily secured another 5 nights with our 40' motorhome in the front campground - Riley Creek. Our 10 nights in Denali allowed us to see 'the mountain' 7 of 10 days and we saw every big animal that lives there over and over again. It was a terrific experience to stay IN the park rather than outside.
All summer we used public campgrounds or boondocked in wonderful spots. Yukon and Canada's provincial parks are great, too. We had no problems finding sites. If you want RV parks (just gravel parking lots) then plan to pull in around 3pm.
I would definitely advise that you don't make reservations for your whole trip. You never know when things can happen to throw you off schedule - roads, floods, fires, repairs or just because you want to stay longer in one area than planned or want to leave earlier than planned. These all can happen on this long trip!
Full-timed for 16 Years
. . . Back in S&B Again
Traveled in a 2004 Newmar Dutch Star 40' Diesel