Not trying to talk you out of getting a new road bike, but you don't need a roadbike to do RAGBRAI. You'll be surprised at all the mountain bikes, hybrid bikes, homemade bikes, 3 person or more tandums, hand pedaled-bikes etc that do RAGBRAI each year. The key to long distance touring is comfort, comfort and more comfort. Why not just your current bike if it is comfortable to ride? The pace of RAGBRAI is fairly slow, so much that hard core roadies will do extra distance each day to challenge themselves.

Best wishes regardless and I hope it works out great...