The Mackenzie Highway from the Alberta border through to Yellowknife, NT follows along the northwest side of Great Slave Lake. It's 2 lane and paved all the way. Some stretches, due to extreme weather conditions, will be rough, likely with some broken pavement due to frost heaves but I would suggest that it wouldn't be a problem to take a coach and toad up the road as long as you mind your speed.
The Liard Highway, as far as I know, is still a gravel road and in the summer is brutally dusty unless you drive it in a rainstorm and then it's brutally muddy! I wouldn't take any RV on that road simply due to the fact that you'll be cleaning out dust from now till eternity. Perhaps someone who has travelled it within the past few years will chime in and offer a more up to date recommendation.
Either way, I would suggest you might want to contact the Government of the Northwest Territories to get current information and literature on travelling to one of the most unique places on the continent. Lived there for 18 years and loved every minute of it.
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