Complex sentence: The first "official" overnight Valley Forge Encampment in connection with a pilgrimage was in 1948, prior to that there were many unofficial campouts on the site as the area was relatively rural and unpopulated.
Split into simpler sentences: The first "official" overnight Valley Forge Encampment in connection with a pilgrimage was in 1948. Prior to that there were many unofficial campouts on the site, as the area was relatively rural and unpopulated.