Book "accessible" tours with Princess, but ALWAYS call the excursion desk directly and tell them you MUST have a lift van or bus. Re-state that you cannot walk, stand, or transfer into a bus or van with steps.
Once you get on the ship, go to the excursions desk as well and tell them the same thing. They will be surprised and of course will not know about your previous communication with the cruise line. Restate the above and be SURE you are getting a lift van or lift bus. Double check again the day before your tour.
We did all this on Princess, and with a lot of struggle and insistance, got accessible tours in Juneau and Sitka. In Skagway we took the train (which has one accessible car) and it comes right onto the dock. There are accessible shuttles here (about every 6th shuttle) into town too. Let the shuttle director on the dock know that you need one.
Do take a manual chair for use on any tenders. We tendered in Sitka and would not have been able to get ashore with a power chair or scooter.