A rest stop is a state park It may be owned by the state, but how is it a park? I'm just talking about rest stops that you stop at to go to the bathroom or whatever when your on a long trip.
The places I've stopped at in Pa. have rules and regs posted just like any other state facility. I don't read them cause I've just never had that on my list of places to hunt or sleep overnight. I've seen similar signs in Ohio and West Virginia.
If yours are anything like ours...I frequently see the state police driving/sitting in them and almost always a maintainence worker there with the exception of a few hours after dark.
Can't hurt to ask someone that may be there on duty but I wouldn't stick my neck out and make phone calls. You'd get a big NO for sure.
A LOT of retirees are packing metal detectors in their motorhomes, as they spend their golden retirement years hopping from one KOA ground to another. And I suspect they don't hesitate to hit the highway rest stops, in all 50 states. I doubt anyone really cares, unless you were leaving a mess. Is that to say that if you asked high enough up the ladder to stuffy offices in the state capitol "can I metal detect at YOUR rest stop?", that you'd probably find someone to tell you "no". But that is also true of EVERY public place. If you keep asking long enough, and hard enough (be sure to mention "digging" and "ARPA", etc... lest they not fully understand), that you'd get no's for every place. But odds are, no one really cared less, nor would have ever paid you mind. Just stay away from obvious historical monuments.