Bebelee, I think the question is what do you want to see the most. If it is Alaska and northern Canada, I would take interstates until Canada to get closer to where you want to be. Alaska is not someplace you will probably go to very often, so spend your time there. I don't like interstates either, but they serve a purpose and that's to cover space quickly.
You can get the Milepost in book stores or order online. You should also invest in some books about Alaska and Canada such as Frommers and others so you know what you are looking at. Last year we took 8 weeks, other times 6. Even with 4 months, you can't cover everything. It is not just Alaska, you will spend a lot of time in Canada—and it is a great place to visit too.
We have traveled to Inuvik in NWT, but it's about 500 miles north of Dawson City, Yukon, on the Dempster Hwy and don't tow a trailer when it's wet—wait for it to dry. By September, you can see the Aurora and that's worth doing—we saw it near Yellowknife, NWT, and on the Dempster Hwy at Eagle Plains.
You can travel via the Alaska state ferry to see SE Alaska, but you will have to back your trailer either through the ship, up the ramp, or both. People will be hurrying you and possibly giving you different directions on the ferry about where to park. It is possible to schedule so you can spend several days in various towns such as Haines, Skagway, Juneau, Petersburg, Ketchikan and Wrangel, though probably not all of them. The schedules are online as well as in the Milepost. If you are bringing a trailer (or not, but have a vehicle) it's good to make reservations well ahead of time. You can drive to Skagway and Haines, take the ferry or fly from there without vehicle and visit Juneau.
This is a very long trip. From Colorado, we have driven around 10,000 miles average on each of our three trips. Makes sure both vehicles are in very good shape. New or nearly new high quality tires will save you problems as they are less likely to fail on dirt and gravel roads. Bring lots of tools, tire repair kit, compressor capable of higher pressures (the kind that plug into the 12 v. sockets aren't powerful enough; direct attachment to battery is better), and extra gas. If you heard the Alaska Hwy is gravel and will destroy your vehicles, that's a myth; the road has been paved for decades. The Cassiar Hwy through central BC is an alternative route, also mostly paved now. There are plenty of dirt/gravel roads and you do ok if you drive carefully, slow down when opposing vehicles are coming and expect some dings.
There are quite a few threads about Alaska. Do a search. Last year we did this one: http://www.airforums.com/forums/f42/...0-a-64080.html