Doing a long trip on a bike is absolutely the finest experience you can have, bar none. It is absolutely addictive, and you'll find that when you aren't on a trip, most of your thoughts will be planning another as soon as possible. It is the ultimate expression of independence, and you'll soon lose track of the number of guys that you pass on the street that have gazes of longing and envy on their faces as you pass by.
You are smart to ask for advice, but ultimately, what you take will depend very much on what kind of traveler you are. Some like motels and hot showers, and some like the more rustic approach. The credit card and cell phone only club is not bad, but I like to have more stuff with me, because riding solo on our bikes lets you be very self sufficient if you so choose.
In any case, you need to have the bike in great condition as an absolute, because getting there safely is the top priority of any trip plan. In this spirit, I always wear ATGATT, but that is a personal thing. Beyond that, there are lots of things you may find helpful. Here is a random list off the top of my head based upon my own needs, which may be very different than yours:
I couldn't bear the stock seat, but every butt is different. I would do a long day trip prior to your big trip just to be sure.
Extreme tape, also known as silicone tape, is sold under various names, and sticks to itself with no gummy adhesive in very high and low temperatures. Great for many emergencies.
Monkey butt powder is great for sore butts and in your socks in the morning. Merino wool are the only way to go for these.
Rain gear is a must, as it will inevitably rain sometime.
I like the waterproof stuff sacks that have the straps and air evacuation system to compact them up a bit. Rok straps to hold stuff on the bike, and one of those insulated backpacker cases for water bottles. I attach that on the top of my tailbag for my Gatorade bottles.
If you haven't procured a sleeping bag and pad yet, I highly recommend Wiggys bags, as they are the best at moving moisture that I have ever used, and I've tried them all. The Neoair Trekker pad is awesome under the bag, and has made sleeping on the ground bearable again.
I would check your brake pads and tires carefully, and if they are close to the end of their lifespan, replace them rather than trying to squeeze just a few more miles out of them.
Tire gauge, naturally, to be used every morning.
I carry a spray bottle of eyeglass cleaning solution for both my glasses and visor, with several microfiber cloths. Clarity defog solution for the inside of my visor, which gets put on every day.
I also carry a full tool roll, but only because I do all my own wrenching, and it doesn't take up much room.
The DeLorme Gazetteer state guides show all the roads, even the little dirt ones, and I consult these when building my routes to avoid as much traffic as possible. I carry all the maps I may need for a trip in a tankbag, and also stuff in several big ziploc bags. Eyedrops can be wonderful after having a helmet vent into your face all day, and I carry extra sets of earplugs in case I drop one or they get nasty.
Although I carry a cell phone and sometimes a handheld hiking GPS, I only use the GPS to tally miles each day, and not for actual navigation. The cell phone can do that, but I personally find all of the electronics to detract from the riding rather than enhance it, so I have it along for emergencies or to find a campground or place to eat, but otherwise leave it off.
Get off the bike every couple of hours, and take lots of photos. I always come home and regret not having taken more myself. Most of all, stop at every possible small mom and pop store that you see and make an effort to strike up a conversation with everyone you encounter. This is the best part of traveling, and what makes riding a bike on secondary roads so interesting. It seems like every elderly guy I have ever met once had a Bonnie, and they love to talk about it, and the longing to be back on it again is there in their eyes if you look carefully. Most of all, don't miss the joy in the journey because you are so focused on the destination. It is easy to get into the "just a few more miles" mentality, and not stop to smell the roses.