The TransAm intersects with the Underground Railroad just south of the Ohio River ferry at Cave In Rock, IL. You could take that northeast into Ohio and leave the route to head east into PA. You can also get in touch with the bicycle coordinators for the states in which you will be traveling through and need routing. Many have online resources as well as printed materials. Nearly every state publishes a bicycle map of some sort that they will send out for free and the coordinators often have more information they can distribute for no charge as well. And while the maps aren't as detailed as ours, they generally offer suggested roads for cycling through their state. Here is a link to the contact information for all of the bicycle coordinators and some online materials: http://www.pedbikeinfo.org/data/state.cfm
For more tips and ideas on how to create your own route, see this blog post on the topic:http://www.adventurecycling.org/resources/blog/how-to-create-your-own-route/
And once you have an idea of routing, you might want to solicit feedback and refinements from the Forums community of cyclists on our Forums.