FWIW,
- As noted, lots of pullouts to use along the way.
- Except for a holiday weekend, most "campgrounds" don't get too busy. If there's only one campground with power/ water etc for 200 miles, it might get hit heavy, but most of the power only or "just a spot" CG's aren't heavily used as the norm.
- As MORSNOW indicated, avoid Seward during 4th of July. FWIW, in general, Alaska CG's near to what passes for population centers (Los Anchorage, Squarebanks, Seward, Talkeetna, Kenai) tend to get pretty ugly around Memorial Day, the 4th, and Labor Day (Memorial and Labor days depending somewhat on weather). I think Seward's the worst, but I avoid camping near Talkeetna and Fairbanks over the 4th as well.
- Purely anecdotally, an increasing number of the pullout spots on the road system in Alaska are getting some sort of signage that purportedly bars "overnight" or "camping"....don't know if it's being enforced or not.