The 12'8" clearance may no longer apply. They have been working all summer in repaving the road, and that is so the clearance may be around 12'7".......probably not what you wanted to hear.
I don't know to what point they measure the clearance height, but hopefully it is at the edge of the road. If that is so, then the towards the middle of each lane should be fine....that is your tallest height with the AC units and such.
I see plenty of 5th wheel trialers traveling the Drive, and I believe they are taller that Class As.
Check post # 5 here: Blue Ridge Pkwy .. 12ft high MH ok? - iRV2 Forums
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263