Southfork was a TARDIS, it was bigger on the inside than on the outside. The interior of the house existed in a different dimension, so the gym could've been anywhere. This also explains how Miss Ellie changed faces. Being a Time Lord, she regenerated into a different form. However she maybe preferred her previous incarnation & regenerated again taking on her original form. On no account tell the producers of Doctor Who this as Warner Brothers who acquired Lorimar back in the late 1980's may get sued for breach of contract!