If you look on the Alaska/Canada forum on this site there is lots of information for you. Folks really start talking about it after January. There will also be an ongoing Alaska 2017 post that folks continually post on even during their summer trip. Check it out.
Can you spare more time than 6 weeks. It's a long, expensive trip. How often do you think you'll go? Most people leave the end of May and return by Labor Day. Early May can be pretty but a lot of places will still be closed and ice will be on the lakes.
We did a lot of dry camping in public parks and boondocking in awesome areas. We rarely used an RV park. If you do, pull in early afternoon and you'll get a site without reservations.
The only reservations we made for our 40' motorhome and Jeep for the whole summer were for the July 4 weekend and for 5 nights in Teklanika campground in Denali Nat'l Park. For those we only made them a couple weeks prior when we could better judge when we'd be there.
Purchase online the Milepost; Mike and Terri Church's 'Alaskan Camping' book which includes Canada and the Yukon and the TourSaver 2/1 coupon book (one glacier cruise will pay for the book).
We crossed into Canada at Sumas, Washington - a smaller, easy crossing.
Allow time to explore Canada and the Yukon, too. Lots to see even before getting to Alaska.
Have fun planning!