Since you're planning to do several 6-month trips, are you planning to concentrate on specific areas of the country for each of those trips? That way you might be able to spend more time and really explore in depth. For example, spend the first 6-month trip only in the east, another one only in the northwest and another only in the southwest - something like that, anyway.
You mentioned an interest in the Oregon Trail. If you were to focus on one region per trip, you could consider following the entire length of the trail - as much as possible, anyway. That would mean starting out in Iowa and Nebraska and following it across the plains all the way to the northwest coast, with side trips to Yellowstone, Glacier, the Black Hills, etc. Similarly, following the Santa Fe Trail in the Southwest (with which I'm less familiar) could involve side trips to the Grand Canyon and southern Utah national parks.
As far as the East goes, definitely I'd recommend Gettysburg plus several other battlefields. A great spot for history buffs is the Williamsburg-Jamestown-Yorktown area in Virginia's "Tidewater" area; base camp in one spot but you could spend a full week or more in the area, absorbing a combination of colonial-era, Revolutionary War and Civil War history all around you. The Boston area is also superb for history, as is Philadelphia. If you are spending a lot of time in one region of the country, I might suggest you check out Bethlehem, Pennsylvania - it's not on a lot of "major history" buffs' radar, but is full of colonial-era history from one particular point of view, and is a town very dedicated to celebrating its history.
I concur with the comment about the Tundra perhaps being less than optimal as a 5th-wheel tow vehicle, unless the 5er is a small one. Nevertheless, I have seen a few towing 5th wheels, and I hope it works out well for you.
As far as planning goes, be aware that many campgrounds and tourist services in the northern states will close by late September or October, and may not reopen until mid-May or later, so be sure to inquire about opening and closing dates, limits on services off-season, etc.