Rivnuts from Homestead, PA on 9/23/2020 8:29:32 PM:
The obvious places on the GAP would be in Ohiopyle and in Cumberland. There are parking lots in both towns where you could park an RV if you are not looking for hookups. Similarly, you could park an RV in Williamsport also. If you need hookups, then you will need to check out the various public and private parks along the way.
There is state park in Ohiopyle with services that accepts RV's. You'd need to make a reservation to do so.
Others may well have some other recommendations.