I have a Class B van, 21 feet long. Could I bring it to watch movies?

Yes, just park in the back row so that no one is behind you!!

Yes

Might be a bit of a pinch but you might find a place without annoying everyone but I don't think it will be a popular vote

Yes, just park in the back so you don't block the road ways. Bring sports chairs too if you want to sit outside under the moon and stars. Many people sit outside of their vehicles too. Just go and have fun.