I do this in Microsoft Streets and Trips, because it is faster to work locally and easy to keep modifying the trip, but ant trip planner will work.
Put in your starting point and destination, and other places you would like to see. Look at the route you get. Get travel guides, tourist information for the places you can go through, add them to the trip. When the trip starts using more time than you have, start prioritzing and weeding.
Keys to planning are getting ideas about what you might want to see (I use library resources, Internet tourist guides, my three-four foot stack of travel and road trip books), and having a tool that can put together routes, travel time, stopping places, and stop times, to fit the road trip into time available.
Years ago I did this with AAA tour guides and maps, these would still work, but S&T is faster.
I think Ohio to California in three weeks is reasonable, because that is 8-10 days driving for the round trip. The problem will be prioritizing and selecting what you want to do along the way, this trip, to fit the time. There is always more to see than time to see it. I've been making the same trip NE Oklahoma to SE Michigan on the average yearly for the past 30+ years, and if I can fit in an extra day to see something, there are enough places to stop, enough routing alternatives, to make it a different trip every time.
