The only reservations we made were for July 4 in Palmer, Alaska and for 5 nights in Denali Nat'l Park's Teklanika campground in August. For those we just made them about three weeks prior when we could better judge when we'd be there.
We had no problems staying anywhere with our 40' motorhome. There are gorgeous spots to boondock right off the highways, not necessilarily at rest stops.
Yes, 300-400 miles/day is too aggressive once you cross the Canadian border, especially if it's your first trip. There are so many neat things to stop for in Canada and the Yukon. Plan on spending three months for the trip, if you can.
Be sure to click on the 'Clicky' on the top of this forum for helpful hints on what to see and do.
Get the Milepost for traveling information and Mike and Terri Church's Alaskan Camping books (includes Canada and the Yukon, also), and also the TourSaver 2/1 coupon book, and that's all you'll need. All available online.
You might consider doing the lower 48 siteseeing on your return from Alaska. For instance, Glacier's highway doesn't really open until May/June. Don't spend too much time on the lower 48 if Alaska is your destination. You'll want to do all you can in Alaska. You could make another trip for the lower 48.
Also, you mentioned Kenai as your destination. Don't limit yourself but instead, drive all the roads you can. You may not get another chance.