As I have explained to DW. We are now retired, ergo don't work. If you don't work you don't get "no stinking vacation" (so my dear old Dad used to say).
But generally we like to travel spring and fall. Leave the summer for the kids who are out of school and need their family time. Weather is generally better and the summer months are for home maintenance and tweaking camping gear, the winter months for tying flys and plotting upcoming trips and research, research, research.
We only make reservations at the most popular parks, and only then when we feel there is no other option as far as getting a spot near where we want. Carry our tablet (with 4G) and have been known to make reservations while on the road.
As with most folks, I suspect, there are exceptions. i.e. this spring/summer. DW wants to spend a week with her sister & family at Gulf Shores first week of June. We'll be in a rented house and camping our way to and fro. Not my idea.