It may be more work than you want to do, but this is the way I do it: I start with a map of the U.S. just to get a rough idea of what is between my start and my destination. Then I break the mileage down into increments of about 200 miles, making note of the large towns and cities at the end of each of those increments and major sights or attractions on the way. Once I've done that I can go to the web sites of the various towns or sights where I'll be able to figure out if I want to stay a day or a while or just one night. Then I go to RV Park Reviews to figure out which parks I like.
This is very simplified. Once you start going to the individual web sites of towns and attractions you find out more about an area and you find other things of interest. Also, when you look at the big picture, you may discover that something you've always wanted to see would be a 150 mile detour in the middle of the trip and maybe you ought to make time for it, either coming or going.
I also use this forum and several others to ask about routes and "best kept secrets" and RV parks. Frankly, I get almost as much pleasure out of the planning as I do from the traveling.
Susan in Music with Mizz Bizzy