I took the ferry from Bellingham to Haines last summer and loved it.
Unless things have changed the ferry's only run every three days, so plan accordingly. The cost was in the $650-$675 range for me and the bike, no room. Camping is first come, first served. Things were pretty crowded at the Bellingham end but thinned out at each stop.
You can sleep on the floor, or if you're lucky, one of the plastic sun loungers in the solarium area, but you'll have zero privacy.
I duct taped my tent to the deck while it was dry and it held for the duration. I got a nice spot by the edge of the solarium yet on the outside, so that meant I got some heat from the lamps, and was protected from the winds that would whip up most nights, yet still had a great view straight out of the tent.
There were a few people sleeping downstairs, in the TV/lounge area which is carpetted, but they had to roll all their stuff up and out of the way during the day.
In hindsight, I think once you've got your ticket you can walk onboard the ship. Go ahead and grab a tent spot and set up, park the bike later.
I used 15' tie-downs to secure my luggage to the bike on the trip, I also used them to strap the bike down once onboard, although they did supply all the rope/twine you needed.
We set sail in the evening. Next day stopped at Ketchikan for about an hour, second day, Petersburg, then Sitka. Last day, made a two-three hour stop in Juneau, but you'll have to catch a cab to get into town as the port is a few miles out. I got off at Haines in the wee hours of the morning. From there the ship went on to end it's journey in Skagway.
The ferry's are quite a bit smaller than the cruise liners so you'll go places and see things those rich pampered bastards never will, the only way you'd get any closer to the inside passage would be to paddle it
Don't put it off...it's a once in a lifetime journey.
Hope this helps, good luck.