We have traveled on I81 near Roanoke a number of times and there is definitely a shortage of campgrounds in that area.
The WalMart in Roanoke is a distance off of I81 (took us about 30 minutes to find it the first time, actually off I581). We have also stayed at the Natural Bridge KOA (a bit north of Roanoke). There are a few KOA's south of Roanoke but we have not stayed there. Dixie Caverns has a campground in Salem, VA just south of Roanoke - little run down but good place to overnight.
BTW, WELCOME to iRV2!
Tom and Katharine
'07 Winnebago Tour 40TD, 400hp Cummins
RVing for 19 years & 150,000+ miles