If you're frustrated with your Garmin in northern B.C., you'll be uber-frustrated with it in Yukon & Alaska. In some spots, you'll see yourself driving across wilderness while you are obviously still on the Alaska Highway. Some of the highway re-routes still aren't updated in the mapping.
The Garmin maps will indicate campgrounds where there a no longer any and then not have the newer ones indicated. Don't rely on the Garmin or any GPS for POIs in the north. One conversation I had with a Garmin tech (about another issue) she exclaimed to me, "How do you find the coffeeshops or stores without POIs in your GPS?". Duh. Their focus was changing to POIs for city folk rather than accurate mapping for those in the wilderness. Must be where the money is...
In Watson Lake last year, one of the campgrounds was closed due to a fire in the main building over the winter. The other didn't open for whatever reason. So only one left. Don't blame Milepost or Garmin, it just was the circumstance.
Might be too early to get up to Salmon Glacier too. Has the snow gone from the road yet?
#1 piece of advice...have a plan but be ready to adapt and shift to a new plan if something isn't as promised.