Distance and time. With a C you could drive in shifts at least as far as the border. You should be able to average over 60 even with stops for gas.
In Canada the speeds will be lower and you may not want to drive at night, but you should be able to get 12 to 16 hours a day if you want to, and average over 50.
After Ft Nelson, things change. The road narrows, and you are in to mountains and curves and hills. As you come to the Liard River the road opens up again and you are out of the mountains, but there are Bison on the road off and on until you nearly get to Watson Lake. They move off the road when they want to. Speed will improve from the mountains but watch the critters.
From Watson Lake to Burwash Landing the road is pretty decent, but the speed limit is still 100kph, or 62 mph, and sometimes less. The road winds up, down and around in mostly low hills and stream crossings.
From Burwah Landing to the border is the perpetual construction and frost heave zone. The road changes year to year but travelers still hate it because it is slow, can be muddy or dusty, has loose gravel, construction and pilot cars. From the border and throughout Alaska the roads are paves but can have heaves. Best to stay at or below the speed limit unless you are really familiar with the road. Speed limit is mostly 60, for a reason.
One thing on a C, check the steering. I don't want to bash motorhomes, but a lot of them do not feel solid in the steering, and steering will be really important from Ft Nelson to Watson Lake. There are fixes available.
Depending on route, weather, traffic and road conditions you ought to be able to spend as much as 20 days in Alaska by maxing out your travel distance per day in the States and lower Canada, and just accepting the road conditions for what they are past Ft Nelson.