Your idea of flexibility is key for that short time period. In fact, you should be able to check the long-range forecast for each potential location you want to visit even before you leave home. You can use that to decide which direction to go first from Anchorage. Then go north, south, or east first, depending on which location has the clearest weather.
And, if its weather is clear, Valdez is definitely worth the time to get there and back, especially since you spent an entire trip on the Kenai. Even the drive over Thompson Pass and through Keystone Canyon is spectacular.
Can you take the rental RV on gravel roads? If not, you may still be able to go to McCarthy-Kennecott by a van tour (it starts in Glennallen and picks up in Kenny Lake and Chitina). It can be a very scenic day trip on your way to or from Valdez. We did it that way by staying two nights in Kenny Lake.
Your problem is time for the distance you would have to cover. So, I would definitely skip Fairbanks and the Kenai and just plan on the Parks Highway (Denali-Talkeetna) and Glenn-Richardson Highway (Valdez-Kennecott). I would only take the ferry from Valdez if you really want to go back to the Kenai. And, if the ferry is that expensive, a flightseeing tour around Mt. McKinley/Denali or a jetboat tour on the Susitna River into Devil's Canyon would, IMHO, be more worthwhile. Both the air and boat tour start in Talkeetna.
If you can use gravel roads, I would still avoid taking it on the McCarthy road. But, you might consider using the Denali Highway from Paxson to Cantwell or the Hatcher Pass road between Palmer and Willow. Both are very scenic and some of the best photos of Denali have been made from the Denali Highway (it was the only road to the park until the George Parks Highway opened in 1973--when I first drove it).
However, all those suggestions go out the window if Valdez is socked in by weather!