We honestly did not encounter 'crowds'. We began our trip crossing into Canada the 3rd week of May so definitely no crowds on the way up. We returned to the lower 48 in September.
We didn't stay in Skagway but instead in nearby Carcross at a public park - an easy drive down the mountain. We also got to Skagway early morning and left by noon before the cruise ship folks swarmed the town. We did that a few mornings which was enough time for us.
July 4 weekend is a biggie for the Kenai Peninsula so we planned so we'd be there in August which was delightful. Same for Homer.
4th of July weekend was one of only two reservations we made for the whole summer and we made it a few weeks prior when we could better judge where we'd be. We stayed in Palmer.
We did make our other reservation for 5 nights in Teklanika campground in Denali Nat'l Park which is the farthest one you can take a RV. We also made that only 3 weeks prior when we could tell when we could be there.
As it turned out we were in the Denali area a little earlier so on a whim we boondocked in a lovely spot not far from Denali and the next morning went into the park and easily secured an additional 5 nights in Denali's front campground - Riley Creek for our 40' motorhome without reservations. We saw the mountain 7 of 10 days and saw every one of the big critters living in the park. There's plenty to do in each part of Denali. It's huge.
The highways were never crowded.
A good publication is Mike and Terri Church's 'Alaskan Camping' which includes Canada and the Yukon, also. It's much better than the Milepost. The Milepost is good for general history and maps.
There are plenty of gorgeous places to boondock along the way. We just used RV parks a few times.