As I understand it, your plan is to "RV" for a week and then begin a return rail/cruise combination?
If you're going to be in AK (on the ground) for a week, follow the earlier post: rent a Jeep or a pickup and just motel it for that period. Not enough time to justify the cost of an RV rental and/or the limitations on where you can take a rented RV.
If the "Inside Passage" is the AMHS, you will see some very neat sights, but again as noted above, you won't have a lot of time in any one spot unless you make several individual leg reservations and stay for some time in the SE cities....which is the way I'd suggest doing it.