First, if you can, please allow a lot more time to get to Alaska. There is so much to see in Canada and the Yukon before Alaska. Three months is the normal time folks take for this trip. It's a biggy and it's slow driving. Sometimes we only drove 50 miles or so.
The only reservations we made were for the July 4 weekend (Alaskans like to camp, too), and for Teklanika campground in Denali Nat'l Park - the farthest campground you can take your RV. For those we made them about 3 weeks prior when we could better judge where we'd be.
As it turned out, we got near Denali early so we boondocked nearby and at 9am the next morning we drove into Denali and easily secured a spot in Denali's front campground - Riley Creek. This was with a 40' motorhome.
You'll have a much more relaxed trip without reservations. Sitesee and stop at attractions as you drive and pull into a campground early afternoon to relax. There are numerous awesome boondocking spots along the highway, too.
Get a copy of the Milepost, Mike and Terri Church's book, 'Alaskan Camping' and the TourSaver 2/1 coupon book. One glacier cruise (a must) will pay for the book. All can be ordered online.
On this site, there's a 'Clicky' at the very top of the page for Hints on what to do, what to take, etc. on your Alaskan trip. Have a good one!
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