I think you're doing it right. Many times no plans = a fantastic trip!
A couple things though - get yourself the Milepost book. It's great for history as you drive and maps. Also Mike and Terri Church's 'Alaskan Camping' book is great. It includes Canada and the Yukon also and describes not only RV parks, but all public parks and boondocking spots. Lots of good information. Lastly, order the TourSaver 2/1 coupon book. One glacier cruise will pay for the trip and you'll want to do at least one.
Lastly, the only reservations we made for the whole summer was the July 4 weekend (Alaskans like to camp, too) and for your stay IN Denali National Park. We only made those reservations about 3 weeks prior when we could better judge when we'd be there.
Otherwise, just start driving and stop when you see something interesting. Sometimes we only drove 50 miles and stopped for a few days of exploring a fantastic area. Having no deadlines is an awesome way to travel. You'll do just fine. Enjoy!
