We're big fans of camping in Yukon & Alaska in late August & September. The fall colours start in early September and are well worth enjoying.
If you're content drycamping, you'll find lots of good gov't campgrounds open until the snow starts sticking to the ground. We usually head out in September to enjoy the quieter roads. As well, the roads are in the best condition of the year in September because most of the repair work is complete.
Some commercial RV parks close right after Labor Day, while others stay open. The campgrounds run by locals tend to stay open later in the season, while the ones run by those who snowbird to the Lower 48 tend to close early.
Only in September will you see colors like this:
Sept 17, 2011
Sept 6, 2012
Sept 10, 2010