You might consider a tailbag instead of the back pack. That's a long time for even a small amount of weight to be hanging on your shoulders.
Sunscreen chapstick and sunscreen for your nose. You may be acclimated as far as the chapstick goes but you may also be riding a fair amount into the sun. Make sure you have some wipes to remove the screen from your hands. That stuff messes with grip rubber and gloves. Also, don't put it on your forehead as with sweat it can get in your eyes. That burns like crazy.
Clear visor and sunglasses or an extra visor if you have the room. You never know when you may need to ride in the dark, although that should be avoided especially for wildlife concerns.
Layout your day with a destination in mind and make a hotel reservation for that night. Waiting for the luck of the draw may find you without a room. You can stop in locally at hotel chains to get directories to review for your trip.
As you're driving, make a point of consciously moving around slightly, changing the way you're carrying your weight. That goes a long way to preventing getting stiff and sore. Make a point of stopping and stretching at least every hundred miles, more often if you feel yourself getting stiff. This is important even early in the ride as once you get stiff, you may not be able to undo the feeling until you've had a full nights sleep. At each gas stop get something to drink and wash down your face, arms, and hands. That helps tremendously to helping you stay fresh.
Also, if you're wearing leather pants, they tend to work well on how they handle the sweat on your ass. If not and wearing jeans, consider an animal skin on your seat. It handles wicking away sweat and is more important than you could imagine for comfort. You could pick up one that's for a touring bike and just lay it on the seat, sliding it under your tailbag or a bugee net on the rear seat, sitting on it to hold it in place. Don't attach it too permanently as you need to remove it and stow it if you get caught in the rain. Once wet, those things take forever to dry and are just as miserable wet as they are helpful dry.
If you aren't riding your bike much, you may want to get out two or three nights a week for several weeks before your trip, about a hundred miles each time, to help get the riding muscles conditioned to those positions. With those considerations, I have ridden as much as 1500 miles in one sitting without any trouble except getting tired from being up 20+ hours.
Carry something to lube the chain with. If you get caught in the rain, the chain lube will be removed completely and will need relubed as soon as it stops raining. For convenience of packing space on trips and the lack of a stand to work with, you might be better off not carrying a spray lube but a small tube of lithium grease. It's a bit tedious, but place a very small dab of lube on each roller, advancing the bike as necessary until you've gone all of the way around. Once riding, it will distribute itself evenly and prevent rust to the chain and sprockets. You may need to do that once or twice, maybe each night, even if you aren't rain riding. Carry that in a heavy resealable freezer type bag along with several paper towels for cleanup and to keep it off your other luggage.
Keep a tire gauge and check each morning before to hit the road. A tire plug kit might also be handy for emergencies as well as the little CO2 cartridges for roadside inflating. The tire plug kits are available at any auto parts store and BMW dealers are known to carry the CO2 cartridges if you have difficulty finding them. It's not that big a deal and I've personally never needed one, but I know those who have.
Consider a route that runs you through Jackson and Yellowstone, exiting the east entrance and heading through Cody. Looking at a map there are two obvious routes to the interstate, one a little more southerly and one a little more northerly. Either is nice, but the northerly one is a little neater to my thinking. I don't remember the route numbers but could confirm them with a look at a map if my suggestion isn't clear when you look at it.
That's all I can think of for now. Hope it helps.