They'll have to stay in Pullen Creek, Mountain View (a dump), or the Federal campground at Dyea that has no hookups. Unless some enterprising RVer decides to buy land and build a new RV park--any takers?
My suggestion--stay in Carcross for one night, spend one night in Skagway, then take the ferry to Haines. In Skagway, we walked up to the ferry office just after they opened, got a reservation for Haines, and left that same afternoon at 3:00. IMHO, Haines is a nicer town than Skagway, has very few cruise ship passengers, has some beautiful scenery (try Chilkoot Lake at sunset!) and lots of bald eagles, and can find some great food. Even better, Haines Hitch-up was one of the nicest RV parks we stayed in on our entire 3-month trip to Alaska--much nicer and far less crowded than anything in Skagway.