There are two options.
1) The Alaska Marine Highway Ferry system - Bellingham, WA to Haines, AK where you can pick up the roads - $369 per person, $70.20 per foot for a vehicle over 21 feet - $4,071.60 for my 22 ft pickup and 36 ft travel trailer attached (58 ft). The vehicle in kept below decks and inaccessible. Cabin if you don't want to sleep in a chair in the lounge - 2 berth outside $411 / inside $353 (bathroom & shower) - $5220.60 for me and the wife in an outside cabin.
Bellingham, WA to Anchorage (Whittier) - $572 per person - $109.00 per foot - x 58 feet - $6,322 - Cabin for 2 - $516 outside/ $462 inside (no private restroom/shower) - Roomette for 2 - $164 outside / $136 inside (no private restroom/ shower) - $7,982 for me and the wife in an outside cabin - $7,630 for an outside roomette.
Note - my TT is under 103 inches wide.
Vehicles - 103-107 inches wide is 125% of the fare - 25% additional cost. Vehicles 108 inches and wider is 150% of the vehicle fare - a 50% extra cost. Towed trailers - overall length is the measurement they use - and they do measure the exact distance when you get ready to pull aboard.
They will also turn off and seal your propane bottles to prevent any usage. There is no electrical power available to keep the fridge cold. Though the temps might be cold - they are usually not below freezing on the ferry route.
As an example of travel times - the ferry for Whittier leaves Bellingham at 6 pm Sunday and arrives at Whittier 10:30 am on Thursday - four nights on the ferry.
The ferry for Juneau/ Haines leaves Bellingham at 6 pm Friday and arrives at Haines at 1:15 pm on Monday - three nights on the ferry.
The good news is that the Alaska Marine Highway ferry and loading dock crews are very experienced with RV. When we did an Alaska inside passage cruise in September 2009, there were over a dozen 5th wheels, class A and TT in the Mendenhall Lake USFS campground outside Juneau, and several lined up at the AMH ferry dock as we went past waiting.
2) Private carrier shipping - which may be on a deck, or may not be on deck. Those have to be arranged via a shipping agent - but you can estimate which will be cheaper. All propane must be shut-off even if you have access to the RV. Will almost certainly not have any electrical power.
US Ship, Lyden Lines, Alaska Car Transport - are all brokers which can give you a quote for shipping an RV.