Grassington's not the best for wilders. Primary School prob ok at weekend as hikers park there. Personally I'd ring the National Park Centre & ask if you can stop on their CP in centre of Grassington (Especially since its out of season).
There's def NO Overnighting at Linton Falls.
You could try Grass Wood Lane Car Park if your van's small enough. (54.082042,-2.024919 i think). Plenty of large gates along this road which get used for parking too.
Theres a Layby just outside Cracoe (54.033083,-2.042411), but be warned the quarry up the road starts early, and the trucks even earlier.