I'm an ad hoc traveler whenever possible.
As a contractor, I don't have set (or paid) vacation days. So I take off whenever I like and whenever the job allows.
Also I travel areas that are "busy" on the off-season. Utah, Nevada, Arizona parks in December/January were great, even trailer village at Yellowstone had plenty of open spots. I'm not a fan of crowds, so most of my summer camping is local, deep in the woods boondocking.
The reservation system games annoy me. Nothing quite boils my blood like driving through a campground that shows sold out online but has dozens of open spots. I don't know if commercial parks suffer this as well, but government parks sure do.