I can give you some feed back based on some of my experiences, the most recent being this:
You don't need a GS unless you want to do casual off roading. Get a touring bike of some kind. It's designed for the job. It should get 200+ miles per tank (typically a 5 gal tank) and will have a more relaxed riding position than a sportbike, making the day after day of clocking hundreds of miles less painful. In other words, do you want to have an enjoyable trip or do you want to go on some rite of manhood across the country where you endure a daily ordeal for a few months?
A windshield is a necessity. A fairing is preferable. Heated grips will come in very handy, as will electric gear. Without them, you will endure some extremely painful rides over mountain ranges. Especially when it rains or snows. And it will.
Get over the idea of sport riding on the trip. With your bike loaded to the gills with stuff, you're not going to be getting close to dragging knee unless you're in serious need of psychiatric aid. That doesn't mean you can't maintain a brisk pace on the twisties, but I did that on my Harley and it's got a more comfortable seat and twice the luggage of a GS or sportbike. And my knees didn't get sore from hours on the road, either. No need to drag my toes on the pavement. I'm not saying you should get a Harley, but what I mentioned as far as comfort is something you should consider. It's a LONG trip.
I planned my route in advance. I checked for gas station availability although I didn't plan each gas stop. I just made sure there'd be gas and planned a few stops on parts that required it. If the bike you get requires premium gas, get some octane boost. You may run into spots that only have 87 octane. I did plan where I would stay each night, either camping or motel.
I personally don't care about the miles per day I clock. That's irrelevant. What matters is hours in the saddle. I like to cap it at 6.5-7 hours of saddle time max. That generally equates to 8-9 hours of travel time including breaks. This gives me time to stop and see the country I'm going through and gets some distance but not leaving me dog tired at the end of every day. Doing that will exhaust you in short order.
I cannot stress the value of taking breaks no more than 90 minutes apart. 5 minutes isn't enough. At least 10 minutes off the bike, not sitting. I often doubled them up with gas stops.
Always carry water. And trail mix or some kind of nourishing munchie.
A tool kit and first aid kit are handy. I didn't need mine. Not even a flat!
I didn't go across the country, but I clocked 8300 miles in 30 days and went as far east as halfway across South Daktota to the Badlands. That's almost enough mileage to go to the East Coast, back here and back to the East Coast again.
Get that expensive underwear made by ex officio. 3 pair is all you need. You can rinse em in the sink, wring em out and they'll be dry by morning most of the time. Don't carry laundry detergent, even in a bag. It'll get out. 3 days of clothing is probably enough. Extra socks, especially thick ones are a must. A rainsuit obviously.
Charge your cell and camera batteries every chance you get. There may be times when you don't have the chance. Possibly for a couple days. Buy extra camera batteries. 3 was enough for me. I hooked up a cigarette lighter adapator on the bike and got car chargers so the bike would charge my cell on the road. The camera charger was AC only. If I could get a car version, I would. I have a 3 outlet adaptor.
Hard bags are the only way to go. They will keep your shit dry, are easier to put on and remove and are more secure. But take them into your hotel room if you aren't camping. Bring a bike cover and use it. A dry seat in the morning is very nice.
Bring micro fiber towels. A couple is probably fine. Hopefully you won't have a bike that needs you to bring a quart of oil. I didn't. I didn't carry a gas can, either.
Sunscreen is a must. You'll get sunburned under the visor. I had the reverse raccoon tan for a month after I got back from my sunglasses.
Pack everything into as few bags as possible. The more bags you have bungied on that are separate, the more likely they'll shift around, pop off a hook and dump your crap on the road. Less bags are easier to load and more stable. Even with hardbags, if you camp, you'll have extra shit bungied on. If not, you can probably do the trip with just hardbags. But it'll cost quite a bit more.
Plan your bike maintenance in advance. You will need tires. Figure out where on your trip you will need them. Schedule an appt. Take a spot 2/3 of the way there and look for alternatives from there all the way to your scheduled stop. Your wear may be more than you estimate. Fall back options are good. You will likely need to have one regular maintenance done. Plan that one and schedule it in advance. Confirm the appt before you leave. If your trip is long enough, you may need more than one. And several tire changeouts, too!
I brought my laptop and posted updates and pictures as I went whenever I was able to get internet access. I upgraded my cell phone plan to unlimited data and used it as a modem. It's slow, but it got me online when nothing else would. And if the motel or campsite had wifi, great. Even better. What was neat about that is I'd post about the last day or few days while it was fresh in my mind. Instead of when I got home as the early days of the trip had faded.
Pack light. Then take 1/4 of what you packed and ask yourself if you REALLY need it. you can buy stuff on the way and you'll want to keep some room for stuff you buy on the way. Every week or so, ship stuff home.
Get your mail on hold. If you're gone over 30 days, you'll need someone to pick it up for you as the post office won't hold it more than 30 days.
There's probably a ton of things I've left out. If you want to check out the thread of my trip, check out this link:
MNB's Big Ass Ride™ (BAR™)
I called it as I experienced it, the highs, the painful lows, everything. The worst being freezing your ass off. In early September. Do NOT ship your winter gear home because you haven't needed it for a week. Repeat, do NOT. Trust me on that one. No matter what the average temps are for all the areas you're travelling in. Half my trip was 20 degrees below average for that time of year. Shipping extra gear home proved to be a painful mistake.
It will be an ordeal no matter how much work you put into making it comfortable. It will be something you take with you for life. It will be a life changing/altering experience. You will meet many kind and generous people. And probably a few dicks driving big rigs, too.
Get a large Ortlieb bag to hold stuff in that you want to keep dry, it's worth the money.
my GSA and would totally recommend it.
