Independent passengers are common, I have sailed Carnival 3 times to Alaska on the Jubilee and had my 9-12-01 Spirit trip canceled. Boarding started around 3pm but last southbound trip with Carnival was 2000. You can't get an exact time for embarkment- it depends on when the ship clears, loading supplies, turning the ship around with trash, cabins, etc. You will be able to board over a several hour span. I never take the cruise transfer. Will you be overnighting in Seward???? If not see the area- or if they want the RV picked up early, use the Trolley to get around, or day a day boat of Kenai Fjords, etc. request the visitor guide
www.sewardak.org