Yeah, don't worry about the reservations as long as you're prepared to boondock it.
Don't kid yourself, campgrounds WILL fill up in the summer. Namely every campground on the Kenai Pen.
That's the only part of the state I found challenging to get a campsite, but never visited Denali in the summer.
FWIW if down on the Kenai near Cooper Landing and looking for a spot to camp, go up to Cooper Lake. Not a bad drive up from the highway and also not publicized. Can fits moderate TT up the road. Beautiful spot on the lake.