The only reservations we made for our all-summer trip was the July 4 weekend when we thought we'd be around Palmer and Denali's farthest campground you can drive - Teklanika. For those we only made them 3 weeks prior.
As it turned out, we were a bit early for Denali so on a whim we boondocked at a lovely spot nearby and drove in the next morning when folks were leaving. We easily secured an additional 5 nights for our 40' motorhome in Denali's front campground - Riley Creek. We saw 'THE' mountain 7 of 10 days and viewed every big animal of the park over and over again. We easily filled the 10 days in the park in both areas.
For RV parks, if you get there early afternoon and sometimes, at any time of day, you will get a spot. The tour groups reserve because they need room for 20 or so RVs. You don't have that problem. I know some folks panic if they don't know where they'll be that night but we don't feel that way. It makes for a more relaxing trip to just mosey along.
We use public parks or boondock as we like that kind of site. There are also many wonderful boondocking spots, some with views of lakes or rivers. We didn't have any problems. We also didn't try moving around on Thurs through the weekend unless we were really in the boonies - never around the Kenai Peninsula.
I'd highly recommend you purchase online Mike and Terri Church's 'Alaskan Camping' which includes Canada and the Yukon, also. They lived in Alaska and traveled back and forth to the lower 48 constantly so they know the parks and areas. It includes great boondocking spots up to RV parks and gives you information on all the surrounding area.
Purchase the Milepost for history and road information, not for camping information. Purchase the TourSaver 2/1 Coupon book online. One glacier cruise will pay for the book and you'll want to do at least one! There has been a special going on for the book - don't know if it's finished or not. Get it before your trip as there might be things you'd want to do before you even get to Alaska.
We rarely use reservations even in the lower 48. That's not the way we like to travel. There are so many things that can happen while traveling and we don't want to rush to get to the next spot. Yes, you can cancel but then it means every other reservations needs to be changed. What a hassle!
Have fun planning. It's an awesome trip!
Full-Timed for 16 Years
.... Back in S&B Again
Traveled 8 yr in a 40' 2004 Newmar Dutch Star Motorhome
& 8 yr in a 33' Travel Supreme 5th Wheel