Busch Gardens has two parks, one in Tampa, FL and the other in Williamsburg, VA.
The one in Williamsburg (European theme) has only one toilet area for assistance animals, at Jack Hanna’s Wild Reserve. Their website does make it clear that they understand what an assistance animal is, and that people with assistance animals are welcome. Though they discourage service animals on rides, they list which rides might be acceptable for service animals.
http://www.buschgardens.com/bgw/pi_accessibility_guide_animals.aspx
The one in Tampa (African theme) does state service animals are welcome, but does not mention toilet areas specifically. Members have reported, however, that there are two relief stations available. Both are marked on the map, though one does not appear to be quite where it is indicated on the map, and the sign is partly obscured so it may be difficult to locate.
http://www.buschgardens.com/BGT/pi_access.aspx