Having done a few long trips, the first of which was 4200 miles within 6 months of starting to ride, what you're wanting to do is possible, but I agree that your daily mileage expectations are optimistic at best.
Keep in mind, first off, that if you expect to be riding interstates on your 250, you're going to be pushing the bike to its limits for hours on end. This is going to be hard on the machine.
Next, 600 miles in a day riding interstate is possible, but if you're wanting to take smaller highways, 350-400 is probably going to be the max you'll want to try to fit in. In my experience, gas isn't always the limiting factor in timing your stops, as your body will get fatigued before you run out of gas, especially in the afternoon. I found myself stopping at about 1.5 hour intervals for about 30 min. "gas stops" on just about every trip.
Now, a couple of differences in my past trips and yours... I always camp (tent and sleeping bag) overnight, mostly because I didn't have hotel money and you can stay at a tent site at a KOA usually around $20, and I've never gone more than a day's ride without having a riding buddy. As for the first, I don't doubt that sleeping on the ground contributed a bit more to my overall fatigue, and it necessitated carrying quite a bit more gear that you won't need. As for having a buddy on the road, it's a double-edged sword. Having someone to interact with can help to keep you awake, and it's nice to have another person right there if you have some kind of problem (mechanical or otherwise), but you're also going to be stopping more frequently. This might be part of the reason we were usually topping out around 350 miles per day, but they were also scenic trips so there were occasional 1-1.5 hour stops just because.
As for what you'll need:
- some clothes. I don't know what you're planning in the way of gear, but I would recommend (in addition to your riding jacket) using a set of over-pants. Something ventilated, possibly with rain protection, but this will allow you to take a single pair of pants for your trip that you won't have to worry about getting wet/dirty on your ride. Just throw on a pair of gym shorts and throw your riding pants on over. I typically do the same with shirts and socks. I'll pack shirts for the destination and two or three days worth of old undershirts and socks. When they get smelly, just toss them in the trash. On the way back, get a pack of new cotton undershirts and socks to replace the old ones I tossed. Saves a bit of laundry (hasn't been available for me mid-trip) and I replace stuff that was at the end of its usable life anyway.
-luggage. Don't plan on throwing everything in a backpack and just wearing it. It will cause extra strain on your body, and you WILL get tired just from riding. There's no reason to increase the level of stress on your body. If you want something inexpensive and waterproof, I'd check out the FirstGear torrent duffles. I have a 40L and it's more than big enough to pack an entire week's worth of stuff, a 25L is probably what I should have gone with. Strap it to the passenger seat and it can double as a back rest.
https://firstgear-usa.com/luggage/tor...-capacity.html
In addition to some sort of bag for clothes, I'd strongly recommend getting a magnetic tank bag. You can find them inexpensively, and they're handy to keep a map, cell phone, snacks (protein bars, bottle of water, etc), and whatever else you want to keep on-hand and easily accessible.
-camel back, or similar. As others have stated, you will get tired and dehydrated. On the majority of my trips I would go through about a liter of 50/50 water/gatorade mix every hour or so. When we were in Arizona/Utah/Nevada/Death Valley in July it was closer to 2L/hour. This is another reason that gas was not always the limiting factor in how long you can ride. (The scary thing is when you're drinking that much fluid and never have to pee...)
-communication. If you're riding with another person, it might be nice to have a bluetooth communication system, but it's not necessary. Either way, I recommend using some form of hearing protection, whether noise reducing ear buds or ear plugs. You'll be surprised how much the constant wind and exhaust noise can add to fatigue. Additionally, it never hurts to have music or an audio book to help keep you awake. Don't forget to keep your phone charged, and it might not be a bad idea to add a usb charging port to your bike. If you've already got a battery tender plug setup, there's actually a usb attachment you can get (or make) that plugs in to the same plug the tender would use, and allows you to charge directly off the battery. Be careful not to leave something plugged in charging with the bike off though, as you could possibly run down your battery.
BORC wannabe 
