If you wanted to have your cake and eat it too, you can do both. You could tow your trailer to Calgary, Vancouver, or Whitehorse, then rent a pickup camper from Fraserway truck rentals. The details can be found
HERE.
You will see many Canadians traveling in rented Adventurer pickup campers from Fraserway on all the highways in BC and Alberta. We even met a young couple from Germany traveling in a very small Fraserway Class C when we were at the city campground in Burns Lake, BC. They had flown into Calgary, where they rented the RV, and had just left Banff and Jasper NPs in Alberta. They were on their way to Prince Rupert to take the ferry to Alaska. I have no idea of the cost of doing something like that, but it is an option.
But, as others have said, a long 5th wheel is no problem in Alaska if you already have it and are comfortable towing it for 10,000 or more miles on two-lane roads. I wouldn't have any problem taking it over the Top of the World Highway, but I personally wouldn't take it on the Denali Highway, the Dalton Highway, or the road to McCarthy/Kennecott.