An update for anyone who is thinking about a trip to Charleston, WV.
First off, the Wytheville, VA supercharger worked well for us in both directions. It was a bit of a scare, since they were working on the power lines when we got there, and nothing happened when we plugged in. They came over and told us it would be 15 minutes until they finished the work, and it was. The person in charge of the work said "I never thought I'd see anyone use these things! I never knew why they put them here." Got a laugh out of that.
Charleston, WV was tough, though. Made it from Wytheville to Charleston with 40% remaining, and most of that trip was downhill. So I HAD to charge, and the hotel's 3 miles/hr wasn't enough given the in-town trips we had to make. I went to the McDonalds shown in most charger maps, and it is with the Blink charging network. I didn't buy a card in advance, thinking that I could just enter the code on the charger screen... and the touch screen was broken. SO. After some stressful time waiting for a call back from technical support, they could activate it remotely, and I charged for 8 hours and was good. I don't know what I would have done if we couldn't have gotten that charger to work. Perhaps the GMC dealer would have let me use (or pay to use) their level 2 charger, but I didn't quite get to the point of asking. My advice if you make the trip is to sign up and get a Blink RFID card in advance.
Despite the stressful 4 hours or so, the trip was a success. I will say that I learned one thing, though: When entering a route via the Tesla maps, the trip consumption graph was SHOCKINGLY accurate. If it said I'd make it somewhere with 40% left, that's exactly how much I had left. It tooks hills and my three-year-old battery into account. I averaged 4mph over the speed limit.
Hope this helps anyone who stumbles upon this thread and hopes to make the same trip!