You should be fine with a couple of Dickensons. Our boat has large, glass windows surrounding the
cabin, so we lose a lot of heat to conduction. The
cabin does not get "warm", but it gets comfortable and the Dickensons keep the humidity down. A 3 gallon tank lasts us several days if we keep it on low and do not run it 24hours/day. We are headed for
Chile now and are curious ourselves how much fuel we will use.
Alaska should be no problem as the
water temp keeps things fairly temperate. Also, fuel is available everywhere so you don't have to worry so much about conserving.
You are in for a treat visiting
Japan and Alaska. Our favorite are was Prince William Sound as it was protected and there were any number of glaciers to visit. You can get an idea of what awaits you by checking out our blog. Happy sailing!