We made this trip a few years ago, but we did it in the reverse order. We were told that almost everyone does it like you planned, ferry up and drive back down. So, according to our source, if you drive up, catch the ferry in Skagway and travel south, reservations are much easier to get because of the fact that there are far fewer people to contend with. We did it that way and had no trouble getting what we wanted. We got off at every stop and stayed 1-3 days depending on what was there. The only place we missed was Sitka because the way the schedules worked out, we would have been there either a few hours or 5 days. We made all the reservations before we left home. The whole trip was a treat, and the only downside was frequent rain.
Bill