When riding distance to a rally, I stopped camping and went with hotels, mainly because of time and comfort.
However, KOAs have nice cabins for a reasonable price, and you can get a discount via your MOA membership.
The
KOA in Crossville, TN is about the only one I am familiar with along your route... but I am sure there are many more.
If you want to get off the slab for some nicer riding, check out US58 east of Abington, VA off I-81.
Grayson Highlands State Park is not far off the highway... it's not long on amenities, but it does have decent campgrounds.
Willville Motorcycle Campground is near Meadows of Dan, VA. Highly recommended.
From there, take the Blue Ridge Parkway north until you need to head east to the rally.
Have a great ride!
Ian
ps -> wouldn't it be cool if there were a web site like MotoCampsites.com to list all the good motocamping around the country?
pps -> which reminds me, check out apps like Campendium and iOverlander. They don't have moto-specific data, but they're good for finding campgrounds on the fly.