Maybe look at: Records of Sherman’s (St. David’s) Union Church, West Manheim Township, York County, Pennsylvania, 1751-1800, by William J. Hinke. It is available at
https://www.familysearch.org/ark:/61903/3:1:3Q9M-C9P2-S3QK?mode=g&cat=265734
assuming you have access to familysearch (free membership). This doc (some of it in German) has Schillings in its first few pages recording baptisms, and these might be relatives. And according to ancestry.com description, Leonhard Schilling appears in the doc as a sponsor to a baptism, although I couldn't find that entry and it may be a different Leonhard. Another thought is to consider others in the church record and/or graveyard circa 1750 and see if you can find out where they came from in Germany. Odd that no other Schillings are in the graveyard associated with that church.
A better source turns out to be:
https://www.ancestry.com/search/collections/4941/?name=_schilling&count=50
Access to this requires an ancestry.com account. But to summarize:
Leonard Schilling appears as sponsor at baptism of Jacob Schilling's daughter, which took place 30 Nov 1755. Jacob and Georg Schilling appears as fathers and/or sponsors of children baptized in early 1780s, with Jacob's last name being spelled both Schilling and Shilling. So Jacob probably Leonard's son, and Jacob probably related to Georg somehow. Look for these two and maybe that will lead you back to Leonard's birthplace.