Listen closely! Scrounge up the BIGGEST tank that fits your bike. I had to ride an 883 to AZ last year, and reserve kicks in at around 90-100 miles with the 3.3. Not good enough, certainly!
Electric vest will remove the misery factor from all but the absolutely worst cold in North America, I consider mine mandatory when leaving town regardless of the time of year. Call me a sissy, but I'll be laughing when it gets a bit snowy at the grand canyon and every one else looks like they want to die!
Windshield! they will stretch your ability to sit on that seat better than almost any other single thing. They help protect your ears, and prevent fatigue from the eighty mile an hour blast of wind you're going to fight for 6-10 hours a day.
Stay off the interstate! Interstates SUCK ASS!!! They are like torturing yourself. Unless you HAVE to make time, you should ride as many backwater tiny roads as possible. A day of 2-lane roads will be fun and memorable, where a day of interstates will be brutal and offputting!
I really don't think you should worry much about arming yourself heavily, unless you plan on searching for crack rocks in every town or stealing all of your gas money,lol! Americans are generally really nice, and with rare exceptions will try to help you out as much as possible. I prefer proper planning, like making sure you don't have to ride long lonely streches after dark, and finding other people on bikes to ride with through bad spots. That being said I usually bring a gun on my rides, but mainly for fun (target shooting in the woods) Never really needed it for anything else(yet)
Start your trip with new (long lasting) tires. I like Dunlop Elite 3's. Many tires will not last more tham 3-5 thousand miles, and buying new ones out in BFE is a terrible idea (trust me, I've had to find tires for riding buddies before out there)
Lastly, take as many big practice trips as possible, before you leave. Any modifications or luggage/gear you plan to bring, test in on smaller ( but susbstantial) rides first. Finding out the gear you are counting on sucks states away from home is no fun.
Everything else is personal, especially since you're traveling alone. When you're with a group, you have to consider the others and be careful to, for example, pack the stuff you use at every gas stop at the very top of your bags, so you aren't holding up the pack, etc. Since it's just you, you can do what you want and take an hour at all stops or 5 minutes.
Oh, one more thing, have someone whom you call every night. That way, if you're stupid and followed my advice to not worry about the gun, at least they'll know where you were last night or whatever. BTW, AAA will tow you 100 miles on basic M/C coverage.
Good luck and have fun! Whatever you do don't wuss out, you'll never regret the bike trips you went on, and always regret the ones you skipped!