Do you have a real valid reason for wanting to go to Yukon so late in the year? The tourist places will be closed, campgrounds and facilities fewer and the scenery not at its best.
I have been to Whitehorse, Yukon during the winter. We ran a company that took cross country skiers to Yukon in March. It was beautiful and I have slept in a snow cave while there, but pulling a trailer would not be something I would consider. I have also visited the area several times in summer/early fall in our truck camper, but it never got cold enough for any freezing concerns. If your trip is in September you probably will not have anything to worry about.
