I think we've all been there. My top horrors were Sequoia National Park, Fort Wilderness and Yellow Stone Fishing Bridge.
It wasn't the fact they were super difficult, but we usually arrive later afternoon to all our campsites and those three had snug fits. Cars all around, extra cars parked here and there. Disney was the worst for cars everywhere. Not a lot of space on the roads and if a group is in the parks there is no one to move their car.
2015 Georgetown Forest River 351DS MH
2008 Keystone Cougar 293SAB 5th Wheel
2007 Jay Flight 30.5RLS 5th Wheel
2006 Jay Flight 29BHS TT