The ferry rides are great for most people but very few take the Cross Gulf run to/from Juneau to Whittier. It isn't a very scenic ride, from what I have been told since the ship runs far off shore much of the time. Just not popular with tourist and probably why it runs so seldom in the summer months and not at all during the winter. You have to have a special reason to take that route. Some people don't want to do the long drive over the Alaska Highway and refuse to fly. So the Alaska ferry to Juneau and south works for them. Then they can take their RV to where ever in the Lower 48.
Mostly people wanting to avoid Canada for what ever reason. If you have a prior criminal conviction such as a DUI and want to take your RV to Alaska, this run will work. You can carry your firearms with you on it and other reasons.
There isn't enough interest in the cross gulf run, in the winter, both lack of tourists and the weather in the Gulf of Alaska. The ride would be much like a roller coaster, in one of the frequent storms, wind and waves. LOL
Most people do the Bellingham to Skagway route or some portion of it. I am not convinced that with today's fuel prices, that the ferry is that much more than driving. Plus, short of taking your own boat on the route, it is about the only way to see South East Alaska amd coastal western Canada.. The ferry is not something we do every trip, as my wife and I both enjoy the drive through Canada on the way to Alaska. We especially enjoy the Prince Rupert to Skagway route. Just long enough, lots of great scenery, the ferry is close in, most of the time, several communities to visit along the way, etc.