Your rig is too long. I recently drove through the park looking for a place for my 34" fifth wheel - not one site tht I could easily pull into. I had a 34' motor home and found only one site I could put it on.
Most of the loops in Watkins Glen have allot of trees and it will be hard to park the unit - only a few sites on Tuscarora Village will work.
http://www.nysparks.com/parks/attachments/WatkinsGlenCampLoopMap.pdf
Try staying at Sampson State Park or Keuka Lake State park - I can give you site number (that is where we stay and drive over to Watkins Glen). Both of the parks are great and not that long of a drive to Watkins Glen. Your camping pass is good for any state park. If you want to walk the gorge - use the camping pass from another park and park in the large lot by the swimming pool area and gain access to the gorge from there.