Camp on the deck of a ferry. Covered solariums located on the upper decks of each vessel are popular sleeping areas and for those traveling with small tents, they are allowed on the upper decks.
The entire Alaska marine highway system is a giant welfare case that causes tremendous financial burdens to the State of Alaska and offers very little benefit. Ticket prices are outrageous.
I travel the ferry system a few times a year.
It is generally a great trip. Weather can change that but the crew is always amazing. Good food and good times!