I would be surprised if state parks across the country agreed to give seniors a 50% discount, but it never hurts to try.
I think it is in Ohio that the state parks honor Passport America Sun-Thurs, whch gives you 50% off.
We have found state parks nationwide pretty ridiculously expensive, and just generally learned to avoid them.
COE, National Forest and National Parks are much more reasonable, and frequently nicer.
PA for an overnight stay is almost always cheaper than a state park, too.
Spend less, do more.