Complex sentence: A major event for BACVA in 2004 occurred in May when they officially opened a totally new Baltimore Visitors Center which was radically different from the old visitors center, which was inside an antiquated modified construction trailer.
Split into simpler sentences: A major event for BACVA in 2004 occurred in May when it officially opened a totally new Baltimore Visitor Center. This was radically different from the old visitor center, which was inside an antiquated modified construction trailer.