I don't know if it will help, but you might want to check some of the NASCAR websites. We were just there this past weekend for the race at Watkins Glen and saw several regular campgrounds but there were also some RV's of various sizes that appeared to be boondocking. We stayed on Keuka Lake which is west of Seneca but at a B & B.You might even want to check with Watkins Glen International Raceway as there were lots of RV's parked on their lots for the race so they might permit camping at other times as well. Good luck and I am going to monitor your post as we would also be interested in finding something similar.
Grew up in that area. The only place I know of is Finger Lakes National Forest between Seneca and Cayuga Lakes in Hector NY. We were in that area last week. Keuka Lake state Park is your best shot. There are a few private campgrounds in that area but I can't think of one I would stay at.
If for over nite parking try the Big Lots parking lot at 5&20 and 14A. I think the Walmart down the road is no ONP
I can't recall anything like what you are requesting in the Lake Seneca area. Way too built up. Other, then well, parking-lot "camping"
2008 F350SD V10 with an 2012 Arctic Fox 29-5E When someone tells you to buy the same rig they own, listen, they might be right. When they tell you to buy a different rig then they own, really pay attention, they probably know something you don't.