I am one of the full-timers who do long-term plans. Here is kind of the thinking I go through. Understand that I have already done a lot of national parks, but I still like to stay at national parks and scenic places, but I am trying to travel a lot slower than in the past. It is cheaper and more relaxing, and I have years ahead of me.
First, I tend to plan my trips partly around family because I have kids in Ohio and California, plus have a lot of cousins in Florida. I think about when would be a good time to be somewhere--like in California when the kids are out of school and the weather is not too hot or too cold. This year, I am leaving Ohio in early May, but my younger son is taking his family to Grand Canyon, so I want to meet him there in mid-June. I take Google Maps and plot out a route that will get me there, while not rushing too much. I look at where I can find Corps of Engineering campgrounds along the way, and block out of an Excel Spreadsheet, so many days at each place. For this trip, it means about 5-7 days at each stop--no hurry.
I also know that my California kids are probably going to Hawaii in July, so I don't want to interfere with that. Getting there in late August would be about right. (My favorite campground in Napa is at the fairgrounds and county fair is not over until Aug 20.) I also know that I want to spend the summer somewhere cool--last summer was really hot some places and I do not like HOT! So maybe higher places, like Colorado for late June and July??? OK, that means after Grand Canyon and Zion, where they are also going to be, I think north and east and look for some interesting places in Colorado. I spent a ton of time online looking for places and found some good ones, but summer can really be a pain because of crowds, so I gave up one place and found someplace else a little more out of the way. (Hint: Campgrounds near big cities tend to really be full on weekends, so I avoid those on weekends.)
So, you really have to first think about what time of year is the best for various places. For example, I want to go to Yosemite, but NOT IN SUMMER because of horrible crowds, but can't go there too late because of snow. Grand Canyon is great in spring and fall, but very busy in the summer--ditto for other big national parks, so maybe May or September/October??? I will never, ever go back to Yellowstone in July!!!!
Anyway, I use a spreadsheet and just generally block things out. Then I do my research and look at climate and crowds. Here is a good place to check weather, by the way.
http://www.usclimatedata.com/When I get confident in where I want to go when, I make reservations. By this point, I will also know what is in the area and what I want to see, but I travel slowly enough that I can still be flexible and not feel hurried.