I want to camp for about 4 days in the Tuolumne campground starting August 12. Wanted to get there on a Sunday morning/afternoon, but the on-line reservations don't seem to have any available until Monday (Aug 13). I know the campground is 50% reserved and 50% walk-in. Anyone know if my chances are pretty good that I can get a camp site for Sunday? I would then just reserve the other days.

I'd say you have a good chance if you're there early. If Tuolumne is full, try Porcupine Flat or Yosemite Creek.