CG Reservations-YNP, GTNP, Glacier, Olympic, Banff, Jasper

We are stuck now wanting to visit the parks you mentioned without reservations and everything is full for the year. The larger parks are hard to visit from campgrounds outside the parks. And I believe most all parks have gone to online reservations at most campgrounds. With online reservations It's so easy to reserve sites a year in advance hoping for a trip the following year.
Your big advantage is the short length you require opens many more possibilities.