How likely to get last-minute spots in national parks

Getting into Tetons should be easy enough without reservations. I've gotten into a Yosemite first-come CG by showing up in late morning (not Fri or Sat). I also managed to reserve 2 nights in Yosemite Valley (one at a time, different sites) just a week or two before leaving (it was July). So just keep checking the reservation sites for open spots due to cancellations, and look at first-come CGs and boondock sites nearby the parks for places where you can't get a reservation.