If you're on a strict schedule because of still at a job and only two weeks vacation, yes, make reservations.
However, if you're retired you have the great opportunity to travel off times. Don't go for weekend trips. Make your weekend begin on Monday.
There are many ways to do without reservations if you're not over pickey. If you need to be in a particular site or a particular loop then you'll have a difficult time. But if you can be flexible there are sites to be had - even in the popular national parks. Do you really need hookups? There are usually dry campgrounds that don't even accept reservations, including in Yellowstone, Grand Teton, Glacier, etc. On the major holidays we never stayed anywhere where there's water - pools, lakes, rivers. That was our time to chill out in a nice quiet setting. We went to the 'water' places after the weekenders left - pulled in Monday early and got our site. Also, there are many city and county parks that are very pleasant. Try national forest sites. They are many times on beautiful lakes or rivers. We also boondocked a lot on public lands - national forest and BLM. There are options out there. Try them!
If we had to make reservations for 6,840 nights during our full-timing years we would have never RVd.