Coast Guard regulations require passengers to move to the upper decks while the vessel they are travelling on is underway.
Sleeping in an RV on the ferry is against regulations so cabins with bunk beds are available on most ferries travelling overnight.
Alternatively, sleeping in any one of the many onboard lounges is acceptable and is free.
Get costs & schedules for travel on ferries in Alaska, BC and Washington at www.ferrytravel.com