That's 440 miles a day, more or less, leading me to proclaim you are more of a man than I am. I try to make it no more than 300 miles a day if I can help it. But to each his/her own.
There aren't campgrounds near Roanoke along I-81 that warrant a visit, although there are some a bit north, which is closer to halfway anyways. Since you are planning long days driving anyway, you'll want places very close indeed to I-81. You might consider Glen Maury Park in Buena Vista, which is a small city park campground. You might also consider the Yogi Bear Campground in Natural Bridge.