The above post gives a good description of what to expect.
Don't think of it as a 10,000 mile trip. Break it down in chunks - daily or every couple days. Then it won't seem so long. We never drove 250-300 miles/day once we crossed the Canadian border. Many days were 50-100 miles. It doesn't make sense to backtrack to see things. Tour as you travel.
Along with the Milepost, purchase online Mike and Terri Church's book 'Alaskan Camping' which includes Canada and the Yukon. It's all you will need for finding every campground, RV park or awesome boondocking spots along the way. They lived in Fairbanks and traveled back and forth so they know everything out there.
Your last purchase should be the TourSaver 2/1 coupon book (online). One glacier cruise will pay for the book and you'll want to do more than one!
The Milepost is good for history and maps. We had it open on our laps daily and read to each other as we'd drive. Don't rely on it for campground information because there are lots more that aren't mentioned. Try to pull into a site by 2pm and you'll get a site easily.
There is so much to see in Canada and the Yukon before even getting to Alaska so don't rush through. Their Provincial parks (similar to our forest service campgrounds) are wonderful. You'll find that RV parks are not like you are used to - they are very basic, not 'resorts' even if they call themselves that.
As soon as we crossed Canada we stopped in the first town and picked up some fresh groceries. We went to the bank and got $300 in Canadian - that's where you'll get the best exchange. That money was used for Provincial campsites as some just had a dropbox for payment. It was also used for laundry and other incidentals. Upon return we used what was left on fuel, restaurants, etc. We used our credit card for a lot of things. We had one with no added foreign exchange charges so you might check yours. American Express is not widely accepted so have another. Before crossing the Canadian border check online for the recent border crossing information on what you can bring across. It changes all the time in terms of fresh food products. Answer their questions with a short 'yes' or 'no' and no 'smart/comical' answers. Be honest and don't try to hide anything. They can pick it up easily.
The only reservations we made for the whole summer was for the July 4 weekend (Alaskans like to camp, too) and for 5 nights in Denali's farthest campground you can drive - Teklanika. For those we only made them about 2-3 weeks prior when we could better judge when we'd be there. As it turned out, we were early for Denali so on a whim we boondocked nearby at a lovely spot and early the next morning we drove into the park and easily secured an additional 5 nights in the front campground - Riley Creek with our 40' motorhome. We spent 10 nights in Denali and saw 'THE' mountain 7 of 10 days and saw every one of the big animals residing in the park - more than once. Some short-term visitors never see them.
As stated, gas/diesel is available but they will be farther apart than you're used to so drive on the top 1/2 of your tank. This is no place to wait for the best price.
There are still old road report horror stories floating around out there. That's history. The roads are mostly paved by now. Yes, there will be areas of construction just like you have in the lower 48 and summer is the only time they can do it. You definitely won't be driving normal highway speeds in many places. The key to not getting damage is to take it slow.
Since you're retired I'd recommend that you plan to cross the Canadian border the end of May, first week of June and plan to return mid to late August. Things start closing down in September.
You'll meet up with the same folks over and over again. The travelers are all going on the same few roads and to the same places. You won't be alone out there. Also, fellow travelers and the locals are extremely friendly and helpful if you should need any kind of help.
Your wife was wise to suggest this trip over a cruise.
Your age and RV don't matter. You'll see everything up there! Check on other forum sites for more Alaskan information, especially after January. That's when folks start getting antsy to get on the road. We took turns driving daily so the other could really see the countryside and watch for critters. Hope your wife can do the same.
There aren't big mountains to cross like crossing the Rockies in Colorado. They are so gradual that you won't even know you crossed them! Have fun planning. It's an awesome trip!