I hope my reply was not too verbose
. I do love Alaska and go several times a year. And here goes another long post!
I usually suggest going to Homer first, then Seward. Just easier to get from Seward back north (less driving). You said Day 4-5 Homer but didn't list any activities. I always like to figure out what interests me in each place, then determine how much time it will take.
In Seward, if you do the sea kayaking that goes to Aialak Glacier, it kind of doubles as a Kenai Fjords tour. So you might not need both (though each Kenai Fjords trip is different and I go on one every time I am in Seward).
Also, note that "hike Exit Glacier" can really only be done with Exit Glacier Guides if you meant to actually get onto the glacier. You can hike the Harding Icefield Trail (a great hike, but strenuous and 6-8 hours) or you can just get to the face of Exit Glacier on your own.
The Kenai Fjords boat trips are excellent. Be sure you get one at least 6 hours or longer. Anything shorter doesn't get out of the bay.
If you take a cruise, you won't get nearly as close to a glacier as you will on the Whittier trip (which emphasizes glaciers) or Seward (a +-30 minute stop at a glacier). I actually love being at glaciers so it wouldn't bother me at all. And if you take a Princess southbound cruise, you get to go to both Glacier Bay and Hubbard Glacier, which is a big plus IMO.
Tough choices re: an extra week in southcentral or a cruise. I have used Alaska cruises as "hotel and transportation" and am out in the port town most of the time the ship is in port. My biggest complaint is that I like more than just several hours in these places. So I generally do land tours now. If you take a cruise, it would have to be days 8-15 since pretty much all AK cruises are 7 nights (or start on day 7).
Personally, I love Denali. But I love the scenery, hiking and wildlife. I am an avid photographer and that also fits well with Denali. I am going there in June for 5 nights. Most people spend far less time there. Fairbanks is definitely worth a couple of nights (great museum, even for a non-museum person like me), large animal research station, Riverboat Discovery, pipeline, and more. If you do this, you might consider returning from Fairbanks to Anchorage via the Richardson and Glenn Highways. Great scenery. Maybe even detour to the Wrangell/St. Elias NP visitor center not too far south of the Richardson/Glenn Highway junction. You can't drive RV's along the McCarthy Road, but there is a shuttle bus and a flight you could take into McCarthy. This is a much less visited NP, and the largest NP in the system. There is a nice Root Glacier hike here as well as some nice free hiking (Bonanza Mine hike is an all-day strenuous hike that I would like to try sometime).
I have never rented an RV but it is a popular way to go (they are more costly for me who usually travels alone, and there are some places you just can't use an RV like Wonder Lake Campground inside Denali NP). Unfortunately for this summer you missed the sales. Usually RV's are deeply discounted when booked and prepaid about November for the following summer. Be sure to check BBB ratings of the various companies. One is particularly bad. Great Alaska Holidays is one that has a great reputation, but not the only one with a good one.
When I stay in hotels/motels, I like to book in advance. That allows me to know how much I am paying, usually gives me a better value for my dollar, and I don't waste time the day I need lodging driving around trying to find a place. If you don't like to plan your itinerary so exactly, the RV gives you flexibility. The only place that requires a reservation, even with an RV, is Denali NP (unless you want to waste a day or two waiting for an open shuttle bus, which can fill up in advance). If you decide to go the RV route and take two weeks in Southcentral Alaska, I might suggest that you go north first (so Denali is your first stop). That way you have your fixed dates for Denali and can reserve both RV camping spot and shuttle bus tickets. Also, if you stay 3 nights in Denali, you can reserve the Teklanika campground (minimum 3 nights required at this campground). That is 30 miles into the park and usually private vehicles can't travel that far. I will be tent camping at that campground in September for the first time (I normally camp further inside the park, but this time Tek works out better).
Hope this helps.
Let me know if you have any questions. I am sure others will chime in with thoughts too.