We full-timed for 16 years and traveled constantly. That's 5,840 nights. If we would have had to make reservations for all of that we wouldn't be RVing.
We rarely made reservations and had no problems - even with our 40' motorhome. We've stayed in major national parks such as Yellowstone, Grant Teton, Glacier, etc. without reservations. Some campgrounds don't even accept reservations. We spent the whole summer in Alaska and only made two reservations - one for the July 4 weekend because we were going to be in a popular area; the other for Denali Nat'l Park at Teklanika campground - the farthest you can drive a RV. For those we only made them about 2 weeks prior when we could judge when we'd be in the area.
3-day holidays: don't go anywhere where there's water (pools, lakes, rivers). Find a quiet spot and go to those places when the weekenders leave.
We usually moved on a Monday or Tuesday and stayed put for the weekend.
No, you don't need a bunch of reservations. That's way too structured for us.