Does the senior national parks pass work here?

Yes it does. Park admission is free, and camping is half price, with the pass.

Yes the Senior Pass gets you in for free as far as the entrance fee. If you're camping there's a separate transaction and the pass probably wil bring a discounted fée for a campsite. Check in with the office.

Yes, I used mine to enter with 4 people in the car.

Yes

Can I use my pass in Maryland State Parks?

How much does it cost per night to camp

Thanks for your answers. That helps.