Opinion follows since I've never taken a caravan to Alaska, but have seen the summer roads chock-a-block full of caravans:
1) Driving to Alaska isn't the mythic and daunting experience it was 30 years ago. Just get in your rig and head NW.
2) There are (IMNSHO) too many variables to be locked in to the caravan's schedule even (as I'm sure they do) if they make it somewhat loose. As mentioned above, what if the weather clears on the last day of the stay in McKinley? Or...what if the tour boat captain says that it's safe enough to go out but rougher than heck and it won't be fun, but the itinerary says today's the day for the tour?
3) Caravans, I understand, start from various places, but what if, for example, one finds the Kluane area to be spectacular (not as spectacular as Alaska, of course. :) ) and wants to spend 2-3 days around there? Or 2-3 extra days around Whitehorse? In addition to the lack of flexibility that could impair the Alaska portion, what if one finds Western Canada to be scenic and wants to prolong that part of the trip.
Last but not least: Alaskans are generally a friendly bunch and more than willing to help out if there is a broken down RV on the road. 'Course, they'll grumble a bit more while helping if they've been stuck behind 20 RV's going to Homer on a Friday afternoon.