Make sure the rental company knows you want to go into Bosnia. There is a piece of paperwork (it was green last year but who knows if that will change) that will be included with your rental papers. The people at the borders will want that. Depending on the route you take (I drove along the coast and then headed inland - gorgeous views) you can be on major roads the whole way. From Dubrovnik figure on a 3 hour drive each way. Definitely a worthwhile thing to do.