Hi Packnrat. The big trip!!! Congrats and you definitely have time to plan. We just returned from our 1st big trip, cross-country to almost east coast, 10 weeks, 10,000 miles! Our plan was I-40 east with a few native American sidetrips, see Tennessee incl Memphis, Nashville, and Gatlinburg, Smokey Mtns and Blue Ridge Parkway. We spent a week in Wash DC area after adding 2 days. On to Lancaster County for the touristy Amish country, Heshey Pa, and Gettysburg. Enjoyed all of it. Then on to Niagara Falls, Michigan, and back to I-90 westbound. We saw a lot of South Dakota and finally had to skip Yellowstone and Grand Teton on this trip.
What we learned on this trip is plan some downtime as you can't plan every day or travel everyday or two. Don't try to squeeze too much in. We skipped many of the big cities like NYC, Phil, Boston and the whole northeast corner as just not doable in 8-10 weeks. Another time! But what we saw was fantastic and even though we had seen some of it before, they had been fast and superficial visits. We traveled from mid Apr through almost the end of June. We wanted to get thru the south before it got hot, since like you, we don't do humidity. We wanted to come back on the northern route and we couldn't get there much earlier than we did. We caught a lot of rain all over, set a low temp in Memphis in May, and hit some warm days on way back in SD.
So, enjoy the planning, the trip and the people. You have a great place here to get info and I will what we know with you as the rest have.