Wow, where to begin...
I took my Versys on a 4000 mile trip from SLC to Seattle back down to DFW, TX. Some stuff I learned:
- Pilot Road 3s are the best tires you can have for riding in the rain.
- Make sure you have good gear. I rode in an Olympia Phantom hi-viz suit and it did a fantastic job in all weather.
- Be aware that Texas will be hot. Seriously.
- You'll have a way happier trip if you stick to the mountains. Drop down to Tennessee and ride the Dragon, don't do something silly like ride down across Nebraska, Kansas, and Oklahoma.
As for bike mods, I have a full set of Givi luggage (V35s and an E52) plus I added a Bags Connection tank bag. It worked out really well for me and I had room to spare. I ran with a stock shield, but you should know by now what you want. If you haven't got one already, seriously consider a new seat. I bought a Baldwin that got delivered right before the trip and I loooooved it. Also consider heated grips and hand guards if you don't have them.
Keep your daily mileage below the max you can do. You'll be happier on a long trip like this if you don't wear yourself out on any given day. I was happy to ride 9 days of 400 miles each. It wasn't until the 10th day that I didn't quite feel like riding anymore, but had to get home. Trust me, nothing sucks worse than having to ride when you don't want to.
When you get to Texas, hit the hill country on your way down. The most famous roads there are the Three Sisters, but there is plenty of other good riding to be had. It will be a little out of your way, but worth it IMO.
Edit: Oh yeah, as for what to pack, that's up to you. I packed two long-sleeve compression shirts, two pairs of compression pants (wicking tights, basically), six pairs of socks (two wool liners, two medium weight wool, two heavy weight wool), some hiking pants I bought at REI, a couple wicking t-shirts, a thermal pull-over, and a golf polo. The compression stuff got worn every day along with the hiking pants, and I'd just layer on whatever I needed for the weather. I found little individual-use packets of Tide liquid detergent that are designed for use in sinks, so I'd just wash clothes in the hotel room at night. It was mostly just the compression stuff and the sock liners that really got dirty, so everything was easily dry by morning (really it was dry in a couple hours). My clothes, toiletries, and miscellaneous stuff went in a duffel bag that went into my top case, so I could pull it out at stops easily and have everything I needed. One side case carried my motorcycle "stuff" (tools, tire repair kit, air pump, spare gloves, spare face shield, etc), the other carried my summer gear for when I got to TX (jeans and a mesh jacket) since I was riding in the summer and knew it would be in the upper 90s when I got home. The Olympia suit fit back in that same side case when I swapped out for that last day of riding. The tank bag carried my phone, camera, face shied cleaner, etc., basically the stuff I wanted to access immediately.
One last tip, invest in a Spot. At $250-ish for the device and yearly subscription it's not cheap, but it's great insurance if anything happens to you on a solo trip. Plus folks back home can track your progress.