Agreed that September and October are nice times to see the north. Bugs are pretty much gone, fall colors are awesome, northern lights start moving some nights, light traffic, and so on and so on.
If you're comfy winging it for your overnight stops, you'll be fine. Most Yukon gov't campgrounds keep the gates open until the snow flies. Last year that was November.
We camped through Yukon in December twice in a Class C when visiting my father for Christmas. Did just fine, despite temps to -40 both trips.
December 1998 -
http://yukonsights.ca/19981212_AlaskaHwy.htmlDecember 2001 -
http://yukonsights.ca/20011215_AlaskaHwy.htmlThis is October 10, 2013, with elk sharing the highway with an RV
http://yukonsights.ca/20131010_Elk.htmlEarly October is also a good time to see the swans migrating south
http://yukonsights.ca/20131002_Swans.htmlIf it's warm enough for the swans, it must be fine for RVers?