Took the ferry from Prince Rupert to Haines in 2013. Excellent trip!
Make sure you measure the length accurately when making your ferry reservation - they do check. You are, in general, not allowed on the car deck while underway. I believe they do a car deck visit on the longer legs. We avoided any extra long legs by starting in Prince Rupert - dog had to stay in MH, wanted to minimize his wait. Walked him at each stop.
We did reserve a "sleeping cabin" and would again, although many "camped" on deck.
Overall, worked out great for us. Enjoyed seeing the sea life on the trip.
2012 Solera "S"
Calif SF Bay Area