The term stateside was started by GIs during WWII. It meant being in - returning to the US. Being on the (United) States' side of the Atlantic or Pacific oceans. At that time there were 48 states, so there is a misconception that stateside refers only to the lower 48.
Since then two more states have joined the Union. Stateside, by the original concept, means being in - returning to any and all of the United States of America.
It's also known, by those in the military, as "The Land of the Big PX."
Thank you for your service TAP!