We traveled the Alaskan Ferry from Haines to Juneau, and Juneau to Bellingham Washington in 2015. We enjoyed this trip very much and it clearly added to our Alaskan experience. How long you may have to wait for a spot to open up on the ferry (without reservations) may depend significantly on the length of your RV and tow vehicle (if any). The cost of the ferry is also dependent on the length of your rig (they will measure at the dock most times), so don't fib on this when asked.
You might also want to decide before you call on whether or not you want a cabin during your trip. Of course, this adds to the expense of the trip.
Best wishes for safe travel and a good time.