So, a couple of thoughts....
- As mentioned, GAH and ABC seem to have a pretty good rep; I rented from AB years ago and it was a good experience (it was long enough ago that they were only AB then, not yet ABC :) ) and while I've never rented from GAH, I hear nothing but good about them.
- As a more-or-less lifelong Alaskan, if you only have two weeks, I'd suggest (anathema, I realize) flying to Anchorage, renting a car, and using B&B's for your two weeks.
- Alternatively, as suggested, take the AMHS ("Ferry") from Bellingham up the Inside Passage. I would suggest some effort to identify stops on the way. One can take the Ferry from Bellingham "straight through" to Whittier (near Los Anchorage) in the summer, but it's a lot of time on the boat...better to maybe take the ferry partway up, spend some time, and then catch the ferry back down?
- If taking the ferry, I'd again recommend no motorhome, just yourselves: do B&B's or motels wherever you stop and rent a car if need be.