Wing it! (Sorta) Our only reservation was the Alaska Marine Highway (Ferry System) from Bellingham, Washington, which we highly recommend. If we go again we will not take a room but do what is called "sleeping on deck". In a large enclosed and heated space ypu are able to pitch a tent or just have a sleeping bag on a plastic chaise. Showers and facilities are provided and some limited cooking.
The vessel has cafeteria food which was excellent. You are not allowed to use your rig except for the dog. we got some dry ice for our freezer, The propane has to be shut off, and put the fridge stuff in a couple of wheeled fold-up coolers from Walmart.There are ice machines as in a hotel.
Every day a ferry goes north and south, you are able to schedule getting off for a few days at many of the ports, and catch another ferry north. We stayed awhile in Sitka, it was great, just boondocked a little way out of town, same with Juno.
At the end of the ferry route you drive several hundred miles thru Canada to Tok, Alaska. We found the Milepost to be very useful. All about are gravel pits used to build the roads in which we overnighted frequently. The Forest Service Campgrounds were $5.00 a night with dump stations often, we never stayed in a private camp. Denali was wonderful you drive about 40 miles into the park to the RV camp and then must ride a bus to see most of the rest. It is twice the size of Pennsylvania.
We also spent more than a month on the Kenai Peninsula. We got a lot of good advice from fishing gear shops and other spots for local highlights. We took the ferry south from Seward after 3 months total, no reservation was needed. The ferry cost was about the same as fuel cost and driving time 4 nights and 5 days with no wear and tear on the rig. TC's are just pick-ups with a load, not RV's which are not allowed in many towns, you have to get in a campground and take a bus, because of limited parking. We were never bothered. The folks we met in Alaska could not have been more hospitable. Fuel and supplies were readily available.