You have mentioned having 3 drivers, but not the ages of your other 3 children. Most kids these days seem to spend all day on their iPads even at home, but will they really want to do that most days for 40 straight days? The drive may be a major part of your adventure, but will it be for them? What types of things do they like to do--fish, hike, climb mountains, take a glacier cruise tour, watch wildlife, go to museums?
If they are really into outdoor adventures, I would recommend saving some time by skipping Skagway and Haines completely (despite Haines being my second favorite town in Alaska; Sitka is first) and just driving through Fairbanks and Anchorage on this first trip. Spend as much time as you can afford in Denali, Valdez, Seward, etc.
And, remember, fishing in Alaska is about more than halibut fishing; Alaskans spend much more time fishing for salmon than halibut. Just make sure you are not traveling the roads of the Kenai on weekends in late July when dipnetting (for Alaska residents only) is in season!
Finally, assuming your Super C has a generator, you can comfortably camp anywhere you can find a level space along the highways of much of northern Canada and Alaska unless posted as "no camping." That might save you a lot of time staying in RV parks for only one night during your hurried trip up and back. And RV dumps and water are available at select gas stations, some visitor's centers, and many Fred Meyer grocery store/fuel centers in Alaska.