We just returned from a quick trip where we got as far as Kluane lake and did a side loop through Skagway to Haines and Haines junction. I know the roads further on may be rougher but can say the roads we travelled were as good as any secondary roads in Canada with lots of minor frost bumps and expansion cracks but little that was really bad and, like others have said, you can slow down where needed. There is one fairly long stetch of construction on the Alaska highway south of Watson lake with pilot vehicles and very good gravel (messy due to water application for dust suppression). Getting on the ferry at Skagway with a 24 ft fifth wheel was an adventure due to the need to turn the rig on a small platform then back up the ramp and around a 90 degree turn onto the boat. The crew were excellent at guiding us.
The Cassiar highway on the way back was narrow in places with steep shoulders so it requires attention but the surface is good pavement or seal coat with the exception of a short gravel stretch climbing south form the Stikine river and a fairly long stretch of seal coat south of that which is dusty and seems unfinished. We found that traffic all tends to travel at or around 100km/hr so you drive for long stretches with no one ahead of or behind you. It feels very isolated. We got no rock chips on our new truck and no rocks on the windshield and we found it easy to get diesel so we never did run below a half a tank on the truck. We saw bison, caribou, bears, and deer along the roads so do be cautious of that. I would avoid night driving if possible for that reason.
You would be good with either the coach or the fiver but might want a toad or tow vehicle to get around in larger centres like Whitehorse where the campgrounds are a ways out of town.
