you sound like me 20 years ago. My then girlfriend (now wife) and I rode from Columbus Mississippi to Lexington Mass. We were on an overloaded, new (leftover) 1988 R100GS. We had no idea what to pack or take or how to get there. After that trip, I knew I had to marry her.
Definitely avoid interstates as much as possible. The parkways are very nice but do require some concentration. State highways are always, always better than the slab. Especially on a single. The slab can be useful to make up time though.
I suggest you both get Camelback's. Nothing will put you in a foul frame of mind like a little dehydration. Since it's so hot, I suggest you start as close to daylight as possible and get off the road early. Naps in the day are wonderful! She will be much happier without the stress of finding some place to sleep when she's hungry and it's getting dark. You will too of course.
Packing stuff on the rack so she can lean against it will help. My wife spent 900 miles or so on a KLR in Mexico and was pretty comfortable since she could lean back some.
Interestingly enough, I will be riding back from Mass on my fly and ride, an Aprilia Futura I bought in RI. I'll be leaving the 29th or 30th, solo, while my wife stays at her parents with the kids. Other than the first half day or so, I have no route planned. Unless I get a chance to hook up with my best friend from 6th grade who lives in Cincinnati, I don't intend to plan one either. I just want to pick off a few more states I haven't ridden in (ME, WV, OH, IN, maybe IL) This is the first time I'm sort of thinking I wish I had a GPS. But heck, a compass, a map, the Great Lakes on my right and the Ohio river on my left are all I really need.
Oh yeah - mail from Mass half the stuff you think you need to take with you and you won't be too overloaded