The only reservations we made for the whole summer were for the July 4 weekend (Alaskans go camping, too) and for Denali's Teklanika campground. For those we only made them about 3 weeks out when we could better judge when we'd be in the area.
As it turn out, we were early for Denali so on a whim we boondocked nearby at a gorgeous site and early the next morning we drove into Denali. We easily secured a site for our 40' motorhome in Denali's front campground, Riley Creek, for 5 nights before moving to our reservation site in Teklanika for an additional 5 nights. We easily fill our time during those 10 nights and saw every one of the 'big 5' critters, some multiple times. We had beautiful weather in mid-August and saw the mountain 7 out of 10 days.
As far as tearing up your rig - there's no need to do that if you drive slow. This is not an interstate-type trip. We had absolutely no damage to our motorhome or our towed Jeep and we didn't take any special precautions to them. Take your time and enjoy it!