You have Iron Butt experience so you'll probably already be aware of what I'm about to suggest, but the best thing you can do for long range comfort is to get a Russell Day Long saddle
. An uncomfortable saddle can add to fatigue after a while (and thus negatively affect concentration), something you don't need on a LD motorcyle trip. I have ridden with one (on another bike) and was able to ride ride from sun up to well into the night with zero butt discomfort.
As to tires, when I used to do long distance travel on my Honda ST1100's, I followed a recommended technique of using a particular kind of long mileage, bias ply rear tire, with a normal radial front tire. The rear was a Metzler ME880 and wore like iron! Normal mileage on my bike for a rear tire was about 5500 miles, but the ME880 could go well beyond 10k. This allowed me to do a lot of mileage in one trip and not have to worry about being stuck somewhere with cords showing. Not sure if the 880 is even made anymore or if they even make it in a size that would fit the Versys. Tire technology has improved since I rode LD so maybe there is a radial tire made that gets equivalent mileage to the 880 that would fit the Versys.
Given the time of year that you'll be making this trip, another thing to seriously think about is hydration (forgive me if you already know this stuff but it might help others). The IBA has an excellent article on hot weather riding
. Even in the northern most states of the US, it can get seriously hot during the summer. HERE
is a cheap, home-made solution to the hydration problem I came up with that served me well. I only got to use it a couple of times before I had to give up LD riding but it worked great.