To be honest, the only way to completely avoid crowds and traffic is to go to Alaska in September. But you have kids in school and most of the campgrounds along the Alaska Highway start closing after Labor Day anyway. So, since that's not an option for you, it's best to just expect it and, on your short trip, plan on going to as few cruise ship destinations as possible--Skagway, Juneau, and the other Inside Passage cruise ports, Seward, Talkeetna, and Denali NP. But that's difficult to do since no one wants to go to Alaska and miss Denali or Kenai Fjords National Parks.
Just to give you an idea, Skagway has 4-5 big cruise ships every day (1,500-2,000 passengers per ship in a town that has a year-round population of only 650!) while Seward and Whittier have about one per day and Haines only had a total of one small cruise ship for part of one day in the entire 4 days we spent there. No cruise ships is one reason Homer and Valdez are favorites of so many RVers.
As far as traffic, the worst parts are the frost-heaved section from Burwash Landing to Border City (where you really need to slow down), any long construction zone (where you will have to wait for a pilot car), the Fort St. John to Fort Nelson section (where the oil/gas fracking boom has significantly increased highway traffic), and Kenai Peninsula (Anchorage's "back yard") roads on every summer weekend and during the week if any of the major salmon runs are occurring on the Kenai River. You can check construction zones in advance on Alaska 511 and Yukon 511 and the salmon runs on the link you have been given above. The only thing you can hope for with that oil-gas field stretch in BC is that the current oil price bust continues through next summer or use the Cassiar Highway, which likely adds too many extra miles to your short trip. And there is nothing you can do about the frost heave except knowing in advance that it occurs every year in the same general locations (and pay attention to the red print in The Milepost for sections you will be traveling).
But the crowds and traffic shouldn't keep you from going--the scenery and wildlife on the trip are still worth every minute of the delays!