Never camped at the Glen since I live close enough to sleep in my own bed. But I've driven track days there and attended several races so can recommend a few good spots to view. For the 6hr race all of the grandstands are open so you can watch from anywhere. I'd think you'd be best off walking around and taking in a few different spots during the race.
The new grandstand added at T10 a few years back is my favorite. If you stand at the very top of the grandstand you can see a huge portion of the track- looking behind you can see the esses up the hill, the bus stop, T5 exit.
The other spot I like is at T2 on the outside of the track. Great view of T1 passing zone, and watching the cars zoom up the hill 2-3-4.
Taking a quick look at the camping maps, if it were me I'd camp in G1 and walk to some of the nearby viewing areas. Lot of great viewing around there, including the above mentioned 6-7-8. T6 is my favorite (to drive, not necessarily to watch
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