$40/day average might be doable if campgrounds are chosen carefully. Camping can be relatively expensive in the northeast. As I mentioned, NJ state parks are a flat $25 for non-residents. The KOA in Cuddebackville, NY, a bit north of Port Jervis, wants $40/night for a non-hookup site. The Jellystone place near New Paltz, NY is $44. A few years ago I stayed at this place, which is on route. They charge $30 but the owner gave me a deal since the place was only half open when I stayed there:http://rockviewvalley.com/
There is a place off route on the NJ side of the river south of Portland, PA that charges $45/night. When I came down from Maine I was shocked to pay nearly $30/night at a place on route near Caanan, CT. That was in '99. They now charge between $40 and $45/night. At that price, I made sure I spent a lot of time in the hot tub.
Then again, the state park a bit north of Poughkeepsie, NY is only $15/night. South of Philadelphia, things can get expensive in places like Lancaster County, but there may be some relative bargains to be had. Unfortunately, I don't have the map for that section.
OP: If the route goes through or near French Creek State Park in PA, I recommend it. IIRC, it's a bit over $20/night. There is not much around in the way of food, but I can show you a grocery source that might be useful.