You guys should plan on it taking you at least 10 hours to get there. I've made this trip 12 times, and it always takes a minimum of about 10 hours to get there. You are forgetting about fuel stops, weather, accidents, flats, the hills on 81, traffic in Knoxville, etc. Last year driving solo, towing with my father's Tundra trying to maintain 70MPH, it took me a little under 10 hours. I combined all fuel, food, bathroom breaks, and it still took me that long. The venue for this years event is a little closer than last, so that is even better.
Every fuel, food, restroom stop you take will add 15 to 30 plus minutes to the drive. I81 is notorious for problems. We've experienced everything from thunder storms that slow traffic to a crawl for miles, to accidents that stop traffic dead for an hour, flats, overheating, tow rigs losing axles, trailers losing tires, losing exhaust system components, having to stop for fuel every two hours because a FJ40 gets 10 MPG, having conversations with local PD about the street legality of trucks... I'm sure I've forgotten a few.
Despite all the disasters our group faced, we always figured out a fix and made it down and back as a group.
Sometimes the tow rig needs to be towed.