Even with our 40' motorhome, the only reservations we made were for July 4 weekend in Palmer, Alaska and 5 nights at Denali's Teklanika campground. For those we made them about 3 weeks prior when we had more of an idea when we'd be in the area.
As it turned out, we got to the Denali area earlier so we boondocked nearby at a gorgeous spot full of the beautiful fireweed. We pulled into Denali early morning and easily secured a spot for an additional five nights without reservations in the front campground, Riley Creek. Ten nights in Denali and we saw 'THE Mountain' 7 out of 10 days and viewed every large animal that lives in the park. Awesome experience!
For our whole trip 95% of our time we boondocked or dry camped in public parks. We had absolutely no problems finding a spot.
For RV parks you'll want to pull in early afternoon. Have fun planning!!
