Another truck camper / river trip option.
Via road there are only two ways into Alaska: the Alaska Highway (at Beaver Creek) or the Top of the World Highway out of Dawson. You should consider going up via Dawson and the ferry across the Yukon and do the Top of the World Highway. The Fortymile River is designated a Wild and Scenic River and you could do it. Dawson is a great little town. Also consider getting into Tombstone Territorial Park in the Yukon on your way to Dawson.
Coming out of Alaska on the Alaska Highway you could leave as late as late September or early October and still have bare pavement depending on weather. We have done the Alaska Highway / Cassiar maybe 7 times in our truck and once in our camper. Several times in winter conditions.
The Top of the World Highway is only an option when the ferry is running across the Yukon. So leaving late in the season means leaving via the Alaska Highway.