Why not take a 3-month period now and keep track of papers you have needed? As full-timers, we carried everything with us. We never 'hid' a fire box. Have you every seen videos of RVs burning? They go up in a flash and you wouldn't have time to get to your fire box. All you should think of is getting out. Period. We, and a lot of our full-timing friends, kept a small 'get out' box near the front door to grab quickly. It contained our most important papers. We also had our computer backup disks in there. (Another set was always sent updated to our daughter.) I'd worry more about fire than theft. The box went with us when we were in the towed vehicle touring or if we had the motorhome in for servicing.
Be sure to ask any Alaska-related questions on this forum in the Alaska section. At the very top of the forum is a 'clicky' for tips on Alaska travels - very helpful information. Three things to purchase for the Alaskan trip: the Milepost, Mike and Terri Church's book 'Alaskan Camping' which included Canada and the Yukon, also, and the TourSaver 2/1 coupon book. One glacier cruise will pay for the book. You can get all three online.
It sounds like a wonderful year ahead of you!