We last went in 2003 in the summer. We want to go next year but with a wedding in Brazil the end of July we can't leave until Aug 1. Wondering how things are in Sep in Alaska. I checked weather and...
As PA12DRVR said, expect most campgrounds to be closed after the Labor Day weekend. There are a lot of places to boondock if you're used to it.
Also be aware that when you head back south, you'll be crossing the Rockies a couple of times going south in Canada. You'll run into snow and colder temps there in late Sept and Oct. Be ready to winterize, carry what you need to do it with you.