Do you want to see something along the way or do you just want to drive and see it all go by through the car windows? You have to decide how far you can drive each day and if there are places at those points that you would like to visit some attraction. There are plenty of online mapping sites - google maps for one that will give you distances from one point to another and how long it takes to get there. You can start planning that way. Here is a link to a site that will put the map together with all of the campgrounds in an area including parks and private campgrounds.
http://www.epgsoft.com/CampgroundMap/Put in a location and zoom in to see all of the campgrounds in the area with links to both the campground site and rvreviews. Move around the map to see what is there if you go a little further or not as far.
Three weeks total to CA and back to Ohio is fine if all you want to do is drive to CA and then turn around and come back unless you don't mind hard driving long hours each day and likely to do this you will have to take the same route there and back without varying the return trip to see more.
If I were you I would start plotting a route and campground stops right now because you may need most of the time to put this together between now and when you have to put in your vacation request.
